Im not sure what pistons went in the corvette engines, but I'll bet they were flat tops, and even if you bought a good set of heads, they are still pretty large in chamber volume....changing the heads alone would be your biggest power adder, but without boosting your compression ratio, it's kinda wasted money. Try to find out what pistons are in there, and if they are flat top, you can look for a set of GM heads, closed chamber from late sixty's engines...97 to 101 cc's would give you somewhere around 9 to 1 with flat top pistons. That would give you a pretty big increase, and let you use a better cam...bigger cam with low C/R isnt going to work. If you do decide to go with aluminum heads (new), look at Canfield from Competition Products in Wisconsin...they are selling for 1399.00 a pair, assembled, not bare. Go with a mild hydraulic cam, around 224/230 duration at .050, with maybe 112 or 114 LSA, it will idle smoother. Get a Performer intake and use your carb, or get a 750 Holley or Edelbrock vacuum secondary carb, and you'll be there.Dont forget about headers too...

I know you dont want to pull the engine, but it would be much easier that way, plus you can always change pistons that way and know exactly what you have!! (making more work for you!) John