Originally Posted by Arcuden
I would probably go with the late-model two-bolt. I prefer a roller valvetrain and I don't like retrofitting since it is really expensive. Additionally, that late-model block is a one-piece rear main seal which will be better than the old two-piece design.
You only need be concerned about using that 4-bolt block if you're going over 500 horesepower or running at sustained RPM, neither of which sound likely in your case.
A 2 bolt with 3.750 stroke will cause the caps to walk as we have seen this to many times thats why the 400 2 bolts had wider caps that make them more stable with the longer stroke which adds more side load to the caps.
A 383 with Vortec heads should make 500 foot pounds of touque.
Here is a link to look over http://ls1tech.com/forums/showthread.php?t=652566
And on those 1 peice seal blocks you can use a 2 peice rear seal crank with the right adaptor.