You're better off with a power adder..
UCF is right, you'd need stiffer front springs to handle the weight, a different tranny (not always but most likely) and driveline...
Not to mention you'd be a straight line car...a 351 is much heavier...The car wasnt engineered for that much weight up front, and you'll feel it.

 

Physically it would "fit" without too much modification to the body. The 351 and 302 are the same size block and only slightly larger in dimensions than the 4.6. The 302 boss is basically a high compression 302 Block with 351 heads and intake and just add small block 400 heads to a 351 and guess what you've basically got. The latter is what I put into my 85 bronco last year and its running in the neighborhood of 300HP @ 10MPH , an improvement over stock.(Of course those are some pretty conservative horsey's) And I haven't even thrown on the headers and exhaust or drive reduction pulleys yet. Couldn't wait to fire that beast up. Unfortunately the cheapest and easiest way to make big ponies is by going old school and all the neat drive by wire goodies and computer techonology in our cars is not going to be at all compatible. If you consider that 350 horse on the 4.0 is only about $8000.00 or so (which is about what I have into my Bronco engine alone) I'd have to ask, "Why?". I mean is the cool factor and that low rumbly muscle car sound really worth it? SURE THE HELL IS! But that's what 65 Fastback Resto mods are for.
But then again, I'd have to say go for it! There is the fact that it hasn't been done yet, right?

 

According to the September of 2006 Edition of Motor Trend, they will be coming out with the S197 model of the Boss 302 (5.0L) which will be putting out about 390 HP alloy block V8. It'll have regular interior and Shelby GT 500 Suspension. Ford is still working on the details. Here's the article (had to scan it in myself)

Page 1: http://i35.photobucket.com/albums/d1...Boss302pg1.jpg

Page 2: http://i35.photobucket.com/albums/d1...Boss302pg2.jpg