If you're bound and determined to go forward, keep in mind that even the 2V Cleveland head port size is large for a 302 displacement, so for a street-use engine we'd recommend going with the smaller 2V heads for better low-end power and torque. You'll need a special intake manifold that will mate the two engine types. Modifications to the cylinder-head cooling passages will also have to be accomplished. Pay attention to the piston and rod requirements as well, and you'll show up at cruise nights with something different.
Unlike the genuine Boss 302 block, the standard 302 item is a two-bolt affair, so you might wish to consider a stud girdle for the high rpm use expected with an engine with such large flow capacity.
In this photo, a non-Boss...
In this photo, a non-Boss 429 is shown in a '71 Mustang. Notice how the standard 429 heads and valve covers seem diminutive by comparison to the Boss version. Because the FE-series 428 engines were no longer available in the Mustang, the Cobra Jet moniker was transferred to the 385-series big-block engine.
As in '69, 10 different engines were available in the Mustang in '71, with the base engine being the 250ci inline-six. Several versions of the 302 were available, and all 351s for the Mustang in this year were of the Cleveland variety. Also offered in '71 was the Boss 351, which now replaced the Boss 302 and Boss 429 as the top-of-the-line performance Mustang. In November 1970, Ford withdrew all support for Trans-Am racing, so the necessity for a special engine displacing 302 ci was gone. Hence, even though a '71 Boss 302 was slated for production, the car never saw the light of day. The Boss 351 featured a 351ci Cleveland engine with a high compression ratio of 11.7:1. The solid-lifter engine was rated at 330 hp and came equipped with aluminum valve covers. Like the earlier Boss cars, a four-speed transmission and a 3.91:1 axle ratio were standard. Ram Air was featured on the Boss 351, as well as front disc brakes, heavy-duty suspension, and front spoiler. Boss 351 cars had the Mach 1 grille, but were identified by a Boss 351 decal affixed in place of the Mach 1 markings. Again, the Magnum 500 wheels in a 15x7 size were used, this time shod with Goodyear F60-15 tires.
Although the largest and heaviest...
Although the largest and heaviest Mustang yet, the '71 version was still a good-looking car. The Boss 351 here shows the many styling cues that were shared with the Mach 1 for that year, including the grille, lower body side paint, and body side tape treatment. A special decal proclaimed "Boss 351," and the hood for these cars received a black or argent paint job, depending on body color.
The Boss series of Ford Mustang automobiles remains one of the most historically significant types of high-performance cars of any manufacture. Designed to be balanced and fast, the Ford dream of Trans-Am dominance was realized in 1970 when the Boss 302 rode to victory. Although the Boss 429 was a homologation vehicle designed to get the 429 Blue Crescent engine certified for NASCAR racing, in street trim it was delivered to the showroom floor in a relatively mild form. The Boss 429 Mustang remains one of the most underrated performance cars of all time. A few changes underhood turned the street Boss 429 Mustang into a nearly unbeatable barnstormer. Finally, the Boss 351 was a great high-performance car whose lifetime was cut short by the impending factors of emissions regulations and high insurance premiums. Sadly, for performance enthusiasts everywhere and for many years to come, the words "high performance" had a hollow ring in Detroit.