Hey deuces, it all depends on what you are looking for. Alot people will tell you the 4.3 is basically a 350 minus two cylinders so someparts are interchangeable. However the amount of power you can squeeze out of them while keeping it street friendly is limited. With the Gas price is almost insane to be running a radical engine back and forth.
My opinion go with the LS1 if you got the cash, besides for $3500 you can pick up whole motors with accesories and tranny. Where? That I dont know, maybe one of the experts like Redheartbeat can answer that for you.