Frame Up Built Boss Mustang Pro Street 604 V8 740 HP

Some cars are built to be mannered and graceful, spending their lives collecting hushed compliments on neatly manicured show fields - others are forged from sheer brutality. This 1969 Mustang Boss 604 definitely falls into the second category. With one twist of the key, the 604ci V8 snarls to life and decimates any possible misconceptions about this being some show-only cruiser. What started as a true S-code 390-powered Mach 1 is now a street-legal thunderstorm ready to rain on every would-be competitor within driving distance. Engineered to the nth degree and topped with genuine Pro Stock pieces, this 740hp Mustang delivers Boss good looks and serious power with just enough manners to freely roam public roads. Looking for the ultimate Pro Street Mustang? The Boss 604 is definitely a contender.

It's hard to call a bright red car with 33-inch rear tires and a gob of motor sticking through the hood “subtle” but something about this Mustang brings the word to mind. Maybe it's the lack of outlandish graphics or the effort invested in keeping so many stock pieces on the car but, in the Pro Street world, this is about as close as it gets to speaking softly. The Ford sheet metal retains its stock proportions and wears a sleek coat of bright red over its laser straight lines. Body and paint present well from all angles with absolutely no sign of anything that could be labeled as a flaw. Panel fitment is equally nice, offering a high quality first impression.

A second look reveals plenty more to like about this fastback. It's hard to deny Boss styling and this car wears it well. At the front, a stainless-trimmed badgeless grille sits above a clean chrome bumper and aggressive chin spoiler while deeply inset headlights are supplemented by a pair of large fog lights. The side profile remains pure Mustang with “Boss 604” decals on the fenders and a sharp body line that terminates in huge vents located at the edge of the quarters. The greenhouse is filled with like-new glass framed in clean stainless trim. Pony badges reside on both C-pillars while the louvered back window draws the eye towards a satin black deck lid spoiler. A chrome gas lid door is framed by segmented taillights which sit above a chrome rear bumper with a subtle chrome kill switch on the passenger side. Back-up lights in chrome bezels round out the look.

Now onto to the star of the show – the engine. BangShift.com featured the car in March of 2012 and described the powerplant as “a rolling bar fight”. We couldn't have said it any better than that ourselves. From the sheer enormity of the 604ci V8 to the massive coolant lines going directly into the heads, a single glance is all it takes to confirm this engine as lethal weaponry. The core is a 604 cubic inch big block block with 4-bolt mains and cavernous 4.625 bores. That mill is filled with high quality internals like a 4340 steel forged crank, forged H-beam rods, and a custom roller cam from Comp cams. It's a serious foundation and one worthy of the exotic crowning pieces. The focal point is a magnesium tunnel ram built by Alan Root and topped with dual 750 CFM Holley Dominators. At either side, Alan Root A-441 heads frame the intake with equally high-end hardware. The combination of smaller carbs and 9.6:1 compression keeps the car somewhat streetable and happy on 93 octane but don't let the detuning fool you – the 604 still turns out over 740hp and sounds positively inhumane.

Naturally, that kind of power isn't placed in some old undercoated road warrior chassis. The Boss 604 sports a well engineered undercarriage equally ready for show or go. In the center, a professionally built Ford AOD 3-speed automatic takes car of shifting while a Strange rear end with 4.56 gears and a spool puts power to the pavement. The rear end is braced to the point that most dump trucks would feel inferior looking at it and the finished product has been sprayed with a sleek coat of dark grey paint. Around the rear end, a fully adjustable link setup with coilovers keeps the back end firmly planted. As you probably noted in the engine pictures, the shock towers had to go to help wedge that 604 in place. Below, a Heidts Mustang II front suspension props the front end up. The roll cage bars have also been carried down through the firewall for some extra strength. Steering is supplied by a manual rack and pinion while braking is left in the hands of a Wilwood four-wheel disc setup. The traditional fat and skinny wheel and tire setup is carried out by 15-inch Center Line wheels that turn massive 33x21.5 tires out back.

While some Pro-Street interiors deny their track-bred inspiration, the Boss 604 fully embraces it with a stripped down to-the-point cabin. Clean black carpet covers the stock portion of the floor, with two high-back bucket seats mounted above. The seats have an OEM feel, aided by the stock-shaped center console between them. The console has been modified to make room for a B&M ratchet shifter, surrounded by a solid piece of aluminum. From the driver seat, a Mustang dash adds a familiar shape to the interior and fills it with an array of white AutoMeter gauges that keep tabs on the rowdy 604. To keep with the race-friendly theme, there is no HVAC system and a plastic delete panel covers the space that would usually be occupied by a stereo. All resemblances to factory work stop behind the seats where impressively clean metal work takes over. Giant wheel tubs accommodate the rear tires while an elevated floor fills the space between. A body-matched roll cage snakes through, offering both safety and a touch of style. Pop the trunk to find a fuel cell wedged between two roll cage bars. A pair of Optima yellow top batteries round out the space and provide plenty of juice for the Boss.

There are a lot of killer Mustangs roaming the roads but few offer both the power and swagger of the Boss 604. With a 740hp big block chock full of rare pieces, a fully sorted custom suspension, and all the visual appeal of the legendary Boss Mustangs, this is ultimate Ford for anyone who has wondered what one of these cars would look like with all the dials turned to 11. If that sounds like you, don't miss the chance to put your name on the title of the mighty Boss 604.