Simply because of the displacement, yes a 351 will make conciderably more power than the stock 302. But, with those restrictive heads, it will not be that much overal (300 maybe). I think that you should do it, just don't be expecting all that power. Now when you start putting on better components (ie, heads/intake/exhaust) then the 351 will give you gobbs of power, it will MAKE you beef up the driveline, which is a good thing.
There are a number of parts on the 351 that differ from the 302, so the swap will be a little bit more expensive, but i say worth it. You need: engine mounts (i think), flywheel, oil pan, oil pump/pickup/driveshaft, lower intake manifold, harmonic balancer, headers, and i believe that you do need a small rise in the hood (1.5 in cowl should do) because the intake sticks up higher.
The mustangworks (this site) has an article on 302/351 differences as well:
http://www.mustangworks.com/articles...n/351swap.html
[This message has been edited by dinomite (edited 06-27-2001).]