Why? It's the same amount of work to do the SBC, and you won't have to run it as hard.
If the 4.3 is something you just HAVE to do, run the automatic transmission that came behind that 4.3 from the factory or use a manual with the lowest ratio non-granny first gear you can find. That 3.06:1 first gear in the 700R4/4L60/4L60E is the main reason those engines feel so powerful coming off the line. They also like to be slightly overgeared. If your combo needs 4.10s.....go to 4.56s, you'll be much happier from a performance standpoint.
Before someone makes a comment about max performance not being important......ask yourself "If I'm not interested in performance, why the hell am I swapping engines?"