i really dont know if i want to feed the troll like this, but i think theres a few points that need to be brought up (or really, just clarified) here.
1) the factory 351w have been proven at the 600-700hp (and more) level that the op is looking for for decades. we arent talking about a 302 here that will break with a sneeze. obviously if you want to make 800+ or spin insane rpms (or just want the extra 1/8" of bore for a 427), then yes a dart block is a much easier decision.
2) the earlier blocks ARE better than the late-model roller blocks (thicker webs, possibly better material).
3) there is NO factory 4-bolt 351w, and modding one to 4-bolt will probably weaken it rather than help it
4) all a main girdle will do is keep the parts together AFTER it splits down the middle, they dont do much in the stability department.
5) non-roller blocks should tend to be more available than the roller ones (the later was only made for 3-4 years, non-rollers for decades), should not be a huge difference in price unless you're looking for a specific year range (which can make a strength difference, but not one big enough to really pick one over the other unless going for absolute max power, which a dart makes more sense for anyway)
6) the best explanation ive heard for the roller "stress crack" mentioned is most likely because of poor maintenance and overheating in the environment in which those motors operated (ALL roller blocks come from trucks and vans, probably most of which have been beaten on by company drivers for 200k+miles in 3/4 and 1-ton trucks, most 1/2 tons will have the 300 or 302). that said, a non-roller block is still probably a better choice for a 408.
7) unless tfs changed manufacturers AGAIN (i believe crane used to make their cams until they went under a few years ago) those cams are NOT billet, they are cast. the earlier ones (and if they changed suppliers again, maybe the new ones) are fully machined, the later ones are more cheaply made and you'll see the as-cast finish between lobes (and yes, i have both, theres even an old thread on here about it).
8) a 408 (or if you end up with a dart block, go 427) will be PLENTY "torquey," thats still a whole lotta cubes with a fairly long crank throw, and provided he feeds it right, should make power just about everywhere. heads in the 200cc class (tfs 11r 205, afr 205 or 225, hell if you have the coin some high port 240s would be awesome), a super-vic intake, and a good custom cam should get you there without even being really "exotic" these days.
