The 4.3 swap may help, but think on this for a while:
I had a '78 Camaro 350 doing 8 MPG. Of course I tried swapping the carburetor and such, but the only thing I did to it that made a dramatic improvement in MPG was a $100 700R-4 from a salvage yard. I did a shift kit, a new filter, and a lockup kit, plus had to get the driveshaft shortened. Final cost under $500. Final MPG, with no centrifugal advance working in my '78 HEI: 25.1 MPG at 65 MPH. This was the same Q-Jet that was giving me 8 MPG before. This was with the 5" rear spoiler, 215/60R15x7 front, 235/60R15x8 rear and a 3.08:1 axle.
As for the 4.3, I had 2 give amazing mileage. The first was in a '94 Chev C1500 W/T, a full-size, regular-cab, long-bed 2WD pickup with a/c, 4L60E ( same as 700R-4 ) and 3.73:1 axle on 235/75R15x7s. It averaged 23 MPG, with a best of 26.
I then had an '89 S-10 2WD regular cab short bed with every option. I had to swap the rear axle, and then I had to get the speedometer corrected. I consistently got 28 MPG at 70 MPH with this: TBI 4.3 / 700R-4 / 3.42:1 / 235/60r15x8s, lowered 2", tailgate down, and 4" front airdam.
But the performance difference between my S-10 and my Camaro was well worth giving up a few MPG.
A week before I sold that Camaro I finally rebuilt the distributor. I know that had I re-checked the MPG at 60 MPH, it would have showed 27. Put it in a third-generation Trans Am, it would do 28.
Sell your 4.3, reinstall your 350, sell your TH350, install a 700R-4, swap your 2.73:1 gears for 3.08:1 gears, and keep the car fun as well as efficient.