You and I are in a similar circumstance. About ten years ago I had a 380 SBC built for my 67 with a 700R4 to keep the revs down. It is a great engine, however even with Holley Fuel injection it had horrible gas mileage. What's worse at the time I couldn't find a 3.75 crank for a one piece seal block. I was told a simple adapter would allow me to run a two piece crank in a one piece block. Well it works fine for racing but for the street it leaks like a sieve. I bought another crank designed for the one piece block and do plan to have it swapped out but I didn't want t wait so I had an aluminum 5.3 built with mild cam, CNC ported heads and total seal rings. Because I'm sure I know more than the next guy, I had the best local builder put it all together and bolted to my TKX 5 speed. I think I would have saved $$$$ buying a crate package.😒