“If it were me”, and I were running an 8:1 motor in Stock, I’d initially be using whatever was the least expensive legal fuel on the list.

After the car was sorted out and quite repeatable......... then you could experiment with some different fuels to see if there is a performance gain over the cheaper stuff.

Of course, I wouldn’t be messing around in S/St either.
Just put the 383 in the car, with the correct cam, heads, intake, carb and start making runs.
Swap the short block when the new 400 is ready.