The cost could run the course from a few hundred to a few thousand, depending on who does what. To do it properly, I'd set aside about $1000 just for machine work. Begin with getting the main bearing bores round and parallel with each other, then register the block on the mains and cut the decks to square based on the stack of parts you will use. Then bore the block to the pistons you want to use. If you get the block squared up, the heads will sit level on the block, the manifold will sit level on the heads and everything should seal up properly. It may be close as to spending the money to refurbish the cylinder heads you have currently or choosing a new set of first-class aluminum heads. Pro-Filer 195's can be had complete for under $1100. That's the head I would choose for you if it were my job to do so.
SBC 23 Degree Cylinder Heads
Make sure whatever heads you use are drilled for steam holes.

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