I can only say from experience get the biggest unit you can afford because the systems are tunable and transferable and if your limited to the projected output of a 253 that's all its going to be good for.
If in the future you want to force induction or put it on a big block you'll find the system lacking.
My holly 2bbl system would work up to 270 hp . I should have opted for the 4bbl as it would have been more versatile.
I hope this makes sense.