4.3/350 Chevy flywheel interchangeable?

As far as the flywheel itself goes, you can fit it behind the engines of the following vehicles:

CHEVROLET   BLAZER          1995-2004
CHEVROLET   C1500 PICKUP    1995-1998
CHEVROLET   C2500 PICKUP    1995
CHEVROLET   K1500 PICKUP    1995-1998
CHEVROLET   K2500 PICKUP    1995
CHEVROLET   P30             1990-1995
CHEVROLET   S10 BLAZER      1990-1994
CHEVROLET   S10 PICKUP      1990-2003
GMC         C1500 PICKUP    1995-1998
GMC         C2500 PICKUP    1995
GMC         JIMMY           1995-2001
GMC         K1500 PICKUP    1995-1998
GMC         K2500 PICKUP    1995
GMC         P3500           1990-1995
GMC         S15 PICKUP      1990
GMC         SONOMA          1991-2003
ISUZU       HOMBRE          1998

I believe these are the engines with the one piece oil seal, so the newer V6/V8 of the SBC Gen 1 design. I was wrong in my comment in that it is weighted the same between the V8 and V6 engines of this type, so you're good to go there. This flywheel will also not work with the two piece oil seal crank, so leaves out the older Gen 1 SBC's. The flywheel will not fit the Gen III/IV SBC's (LSx based engines), because the crank has a completely different bolt pattern on it.

As far as the NV3500 transmission, this should have the standard old style bolt pattern on it, which means it will bolt directly to any SBC design up through Gen IV (LSx based). The only thing is, the LS engines have a slightly different bolt pattern where one of the bolts is offset, which means you'd only get five bolts located to mount the transmission. This is not an issue as people do it all the time.

The only thing which would be of issue with running it on an LS engine is the spacing of the throwout bearing to the clutch forks. And with that, I'm really not sure how you'd overcome that. With the transmissions usually ran with the LSx blocks, you have a hydraulic clutch slave which sits inside the bellhousing on the tranny main shaft. You adjust it by shimming it. I can't tell you exactly how you measure it and what shims you'd use, but it all depends on how far back the throwout bearing is from the clutch forks.

All-in-all, you can make it work, but it isn't exactly easy. I mean, for some applications, it isn't just a bolt up, but rather it takes some effort and planning.