Or do an all out build.
The Stock SyTy turbos were pretty small and the engines had their weak points as well.
If you're doing a conversion you should just take your 4.3 block and have it built with lower CR pistons, 4-bolt mains, Vortec heads and run a big turbo.
That fixes all the inherent problems you'll get by trying to drop a SyTy turbo or motor into your S-Series.
FWIW, many folks who have built 'clones come to find out they spent nearly as much as the real thing.
Just buy a SyTy and keep your S-Series for a beater.
Best of luck either way you decide to go.