I've got a stock 509 casting 400 2 bolt main engine.

I also have an 89 350 block ready to go to machine shop shortly.
Wanted to make it into a 383 with ~375-400hp at the crank @ 5000-5500 rpm and ~500 torque at the crank in the 3000 rpm range give or take.

Thinking of using the 400 stock crank in the 350 block. It'll need a 2-piece to 1-piece rear main seal conversion (anyone recommend a source for this). Has anyone done this? Is it cost effective? Any problems with this idea?

Later on I want to make the 400 a high horsepower high rpm screamer, so I plan to buy a new crank for it. Don't want to waste the crank, might as well use it in the 383.

 

I haven't done it, but here's a source.
http://www.competitionproducts.com/page61.html

By the time you get your crank machined and buy the adapter, I'd think you could buy a new 1-piece 3.75" crank (assuming you sell the 400 crank you have).