Thermoquad question

OK,
I just repaired my TQ's warped bowl with the bowl from a donor carb. Pretty straight forward. Its a 9014 carb from a 1974 400 cid CA AT car. Trying to gain some fuel economy I just swapped the secondary jets from 5143 to 5137, the rods from 2005 to 2103 and left the 4098 primary jets in place.

I have a pretty stock 318 V8, cast iron stock intake with the EGR blocked off, stock 904 tranny and 7 1/4 rear. The car ran great and I was getting 18 MPG before the bowl warped.

The question is how much of a difference is there running a big block carb instead of a small block carb? Does the 1 3/8 bore for the small block carb make that big of a difference over the 1 1/2 bore for the big block?