That depends with the R/S ratio! A long rod motor isn't nessecarily the ideal combination. Engines that feature short rod to stroke ratio's tend to be able to utilize large heads better and operate more comfortably on pump gas with higher compression ratio's. Like you say however it all depends on the combo? As for Unit's statement about a 347 vs. a 351 making about the same torque, he's essentially correct. If each engine is given a comparable intake and exhaust set-up (air valve, cam, heads, headers, etc). Then except for the spare difference in overall size (and R/S ratio as you indicated creating some frictional difference), they should make about the same power. Where each might excel?
The 351 will definitely be a better high RPM motor by virtue of its better R/S ratio. It also can make use of slightly smaller intake ports, it will also last longer. The flip side; weight penalty, increased sensitivity to high compression, better suited to high octane fuels.
The 347 actually is a better street motor due to its reduced sensitivity to high compression, better sutied to low octane fuels, more tolerant of larger heads (and by extension possibly over camming), short R/S motors also tend to be volumetrically more efficient because the piston is travelling faster down the bore creating a intially stronger vaccum signal. and also the lighter and more compact dimensions. The flip side; reduced longevity, not quite as good in high rpm performance, can't take advantage of the slow burn rate of high octane fuel (which helps at high engine speeds, since the fuel burns slower helping to maintain the pressure exerted on the piston)
However, what somebody really needs to look at is the intended usage, and what your goals are. Personally I find the 347 a more attractive alternative to a 351 from a dispalcement point alone, maybe even upto 370 cubic inches which is achiveable with the siamese bore blocks available (4.155 x 3.40), which can swell to 377 cid at the extreme (using a 3.5 inch stroke) all mashed down into a block the size of a 302. Since this is the upper limit (at this time
) Then anything larger is 351 territory.
Chevy's much vaunted 454 has an R/S of 1.53:1
The 347 by contrast has an R/S of 1.58:1 and only trails behind chevy's 350 which is 1.63:1, by .05 and everybody here with a brand x buddy has heard without fail countless times how much better the chevy stuff is than the ford stuff. How much tougher it is blah, blah, blah!
So to me the R/S issue is pretty much a moot topic and is really just a nit picking issue when trying to justify the 351 over the 302 based engines. Probably what makes a bigger difference in longevity is the location of the pison pin in relation to the ring lands. Push the pin up into the oil control ring and you'll have some oil control problems here (and down the road combined with the higher rate of wear).
And on the case of using a 331 as the max size for a SC'd engine, Eh, maybe on a stock 5.0 block. they tend to be on the fragile side of things, but an SC'ed 347 using an R302 or A4 block (and proabably even the new sportsman block) I don't think there would be a problem. If you build a 347 with a stock cast iron crank that is offset ground, using factory armasteel rods (which are cast) and then using some off the shelf pisotn that happend to fit, then your bound to have problems. Change this to a steel crank, forged rods, and purpose designed pistons then its not so much of a problem.