Looked at 30's plymouth the other day. Guy put in a 350 (lame) but showed me they the reciepts and dyno sheet. Lots of money in the engine, looks like around 560 hp but the CR is around 13:1. Any cost effective ways to get this down so I don't have tear the engine out or buy 100+ octane fuel all the time. Assuming I buy the car.

13:1 in a smallblock chevy is 2 things , maybe a little of both , domes on pistons , small CC heads , change out one or both ... race fuel will be cheaper ...

Thicker head gaskets would help, but probably not enough to make that much difference. Chevy heads are relatively cheap though...

If the car is nice enough and the price is right, I wouldn't let the engine stop me from buying it in any case

With receipts and a dyno sheet, you could probably sell the engine without too much trouble and install a Mopar

Changing the cam might also bring down dynamic compression to let it live on the street.

Probably easiest to pull and sell the engine to somebody who wants a hot 350 and put a milder crate engine in it.

Then engine won't stop me from buying if we agree on a price. I just was looking for a way to make the car more streetable. I can put any sbc in it to mak it driveable for the spring and save up either put a real mopar engine in it or shamefully consider a sbc stroker motor for it, which are much cheaper than mopars.

Look at the receipts and see how he made it "13:1".

Might find that's an optimistic guess and not a measured specification. If it is indeed a fact then you ought to be able to see what was done via the receipts and can undo it. SBC heads are cheap, you could probably pull the current heads, sell them and put on a set of larger chamber heads for not much out of pocket after it's all said and done.