a 383 is just a 350 block with a 383 crank right?..

 

kinda, technically, it's a 3.75" stroke and a 4.030" bore. but in plain english that would be a 400 crank in a 350 block .030 overbore. (3.75stroke/4.00bore is a 377/8 if i remember correctly)

if you put an oem 400 crank in an oem 350 block then you will need special bearings.
however, if you go with an aftermarket crank, you can get a 3.75" stroke with 350 mains.

 

To run a OEM 400 crank in a 350 block, you have to turn the crank down to the right size.

Mw66nova, I think you got that confused w/ putting a 350 crank (3.48 stroke) in a 400 block. They have special bearing spacers for that.

 

A 383 is a 350 block with a 400 crank. To me it is a pointless application as the 400 would create more power with less fitting (assuming identical builds). If you want a 383, then I suggest having the 350 block align bored to accept the 400 crank. If you want to run 6" rods, then you need to mill the block to provide extra clearance.
If you cut the crank down, although it will work, you take strength away. If your heart is set on a 383, then I suggest buying the kit, unless you happen to have the parts just laying around.

 

The origin of the 383 was .030"-over 350 blocks with a 400 crank that was too worn to be turned and used in a 400. The mains could be turned down to the 350 size and the crank was still useable. The other source for 400 cranks was cracked 400 blocks, and 350 blocks being more plentiful.

Align boring a 350 block to fit a 400 crank - I'll have to admit that's the first time I've heard of that. You'd lose more strength from the block than you'd lose by turning down the crank. And, the smaller diameter bearing makes for lower bearing speeds, which is needed for higher RPMs.

These days, the aftermarket has responded to the 383 demand by making new 3.75" cranks, even one-piece rear main seal and internally balanced. Since used/rebuildable 350 blocks are more plentiful than used/rebuildable 400 blocks, it's still more popular than building a 400.

If you're buying a new Motown block, it makes more sense to build the 400. I'd still use the 350 main size, though, for the higher RPM capability of the lesser bearing speed.

 

Very well said. That is also the first time that I had heard of align boring a 350 block to fit a 400 crank...

 

Through the years, we have found more problems with crank breakage than block breakage (given a properly destressed block with girdles and all). Just experience, but turning the crank will be cheaper and it works as long as you don't go ungodly on the induction...we shoved (2) 6-71's on a 383 and snapped the crank once.