Hi, I've got the carb'd 4.9 too.
It actually goes like stink, but if you want to boost it more, here are some issues.
1. You need to put in the Allante valve tree. It is steel and the aluminum one can crack if you are upping the rpm and power
2. The intake is a dual plane intake so it is actually good for a carb.
3. I have been told you can use 3800 roller rockers on it, but the engine doesn't need it.
4. The engine doesn't need much porting. Cadillac did a nice job inside that engine.
5. Adding either a turbo or sc is not cheap.
If I was not limited on budget, I'd be going to Edelbrock and getting one of their roots types and hooking up with the serpentine belt. On the carb'd engine you can mount your alternator low at the front, leaving room for a top mounted sc. I am not sure this is cheaper than replacing the engine with a North* If you already have a donor car with the ECM etc. it will be cheaper than buying a supercharger. The North* does put you back on EFI and all the "fun" of electronic motoring.
[This message has been edited by Arns85GT (edited 11-22-2013).]