possible carburetor swap

Let me start off by saying that I have sold performance parts for slant 6 engines, but I have never personally worked on one. your best bet is probably Clifford Performance, they have a 6=8 Mopar 225 combo that they claim increases power 70%. Who knows how true that 70% figure is, but it should certainly increase power. Comes with an aluminum intake, a 2 barrel Weber carb, and headers. It is here:
https://cliffordperformance.net/store/ols/products/68-mopar-225-coombination
With the Weber 2 barrel, I would expect gas mileage to be about the same, or maybe a bit less than it currently is, but it should certainly have more power.

Offenhauser makes some 4 barrel intakes too, and you can get a set of headers. The Holley 390 CFM vacuum secondary carb used to be very popular with the slant 6 crowd. I believe that the 390 has become extremely expensive lately, over $700...