Dave,
Depending on the engine builders in your area, the balancing and blue printing are something that you can live without if they want to much to do it. I think in your case the 750 Holly Double Pumper carb would be the best choice since you aren't going with big cams & etc. If the heads are stock, having them pocket ported and the runners cleaned up will gain you around 20 horsepower depending on how rough they are. And the pocket porting is a very easy and simple job. I have stainless valves in mine that the dealer out in when they did all the work and they have been great. I believe that in the long run, you would pay less and gain more by using your block and then just having done to it whatever you want. Check the head numbers. Mine came with the 427 heads and LeMans rods and other low end stuff that was different from the regular 428 CJ. If you have 390 heads, try to find some 428 police heads as the 427 heads are getting harder to locate. As for actually knowing the bore, it would have to be measured to see if it is a 390 or 428 bore. Also look at the cam you have and see what it is.
Ron
