I have always heard stories about guys trying to snow ski up hill....?!??...spit into the wind...stand on Superman's cape....mess around with ...er...um...non-turbo V6's.
The reason people swap in 350's, be it an Olds, Buick, Pontiac, or Chevy is the significant power increase with not much more fuel use, given you use the same gearing and transmission. Or, my '78 EL Camino that was V6 four speed manual, got 19 MPG at 75 MPH with the stock 454 I swapped into it with the stock 2.73 rear end gear ratio.
Only hair dryer V6 cars sound 'okay' with exhaust. You could spend all that money and time and its a 3.8 or even 4.1 that makes like, 80 horsepower. Check that 100 for a 3.8 and 110-125 for 4.1 per Wikipedia those being production values(not worn engines with the fuel we have now!). Of course, all the things that make a V8 more efficient will do the same for a V6 with less yield, say a cold air is 15 hp on a V8 it will be like 4-6 on a V6. I took the less often traveled route and my Beater Project '83 Regal just got a '75 Buick 350 and I opted for a TH350 over the 250C (lightweight lockup TH350).
I understand fuel mileage, find the lightest car you can, then stuff in a V6, like a Monza or Vega. G-body is sorta' portly (34-3800 lbs easy) for a 100 hp to push around, it takes 60-80 to maintain 65 MPH with the air on, depending on winds and aero of the car. Do the math, that's taxing the engine at or near maximum output compared to half of the given output of even a smog-dog V8 at 200 hp.
Post up some pics already!