Sterndrive Engineering (SEI) makes an OMC to Alpha conversion kit for $350; it says that it works for Cobra drives so not sure if that helps in your application. They sell complete outdrives too and are worth a look.
On the SBC exhaust side I am running some GLM aluminum manifolds and 4" risers and they have worked out great thus far. I think they came in around $800, but actually only $100 more than the cast iron setup when you figure in the shipping. Certainly easier to manhandle and probably around 80# removed from the rear of the boat.
"Speed's fine, but accuracy is final."
- Bill Jordan
1998 Donzi 21LXR
1971 Boston Whaler 13