The advantage of the GMPP is every part is new. Block, crank, every casting and part is new, not re-manufactured. Is that a good thing... ???? not always. A new block which has never seen heat cycles will move around, casting sand will release (and go through the oiling system) bores settle out of round, and any stress riser not revealed by previous heat/stress cycles will surface (cracks in heads, blocks, cranks, rods, pistons, etc). With production being paramount in todays manufacturing,, 'new' may not always be better than a seasoned used item. BUT, that's not saying every new item out there will fail. Just that there could be that potential. But.... "NEW" is also a 'advantage' of the GMPP also,, it's a virgin 4.000 bore block with at least 1 or 2 rebuilds in it. I'm not sure what piston GMPP uses but i'd assume cast which I'd have no problem with up to 325-350HP
Flip side..... The "Blueprint" engines Summit sells appears to be a decent product. (BTW, your link has an extra http in it) They claim to sonic check bores, use a one piece rear seal block, is an efficient Vortec style head (of un-described origin tho)..... Necessary things they dont include will be into your pocket a little. It is a EXTERNALLY BALANCED assembly. That means you need a harmonic damper and flex plate for a 400. Your old internally balanced harmonic damper and flex plate will NOT work. (the GMPP will have the damper but also not include the flexplate/flywheel) I really doubt they fire and dyno test each and every engine. They claim to "send dyno results" with each engine, but more likely the do just like GMPP, and most others,, they build and dyno test one, then all clones built to same specs get the same dyno sheet. "Theoritically" should be the same,,, Suuuuuure it is.
Good, bad or indifferent.... I think I'd get Slummit to GUARENTEE the bore is 4.030 (and not 4.040 or anything else other than 4.030) and go the Blueprint way. You don't want to spend this kind of $'s and have no usable cylinder wall left for future rebuilds. You can go +0.040 pretty safely. And unless you explode a motor and beat the crap out of a cylinder wall,, 0.010 more should clean things up OK for one more rebuild. The hypereutectic pistons they use are hard,, actually termed 'harder' than forged, and run tighter clearances than forged (but are brittle so keep tuned for NO detonation!!!). When hypererutectics fail it's usually disasterous as they shatter like a pyrex bowl hitting a tile floor And that brings near total engine carnage when they do. (have some cool pictures of a 400 missing #5 piston,, I mean TOTALLY turned to dust and gravel in the bottom of the oil pan). Because of the tight clearances used with hypereutectics, failure to keep up on maintenance and run dirty oil will score the bore beyond what 0.010 will clean up. So there is some risk there if your not good about running a quality oil and keeping it fresh and clean.
There is a 3rd option here that may be worth considering,,, is there any local machine shops that racers tend to use? Having a 383 built locally gives you someone local to deal with in the event you have warrantee issues. And local will save shipping fees which is pretty costly on something as heavy as a motor. The last 3 I had delivered were shipped Yellow Freight and you had to be home ALL DAY on some scheduled week day to receive it, and the charge was like $300 as I remember,, (+ the lost income for the day of sitting on your thumb). With a local built motor you can dictate exactly WHAT and HOW it's built. If you want to forgo the risk of a flat tappet cam and have it buiult with a roller package,, it's all up to YOU.
just my 2cents
