Tank on my 69 holds 19. I learned the hard way, but at least she went dry in the driveway.
I put a Proform upgrade kit in my HEI last weekend. got it from Jegs. Made a huge difference all around. Of course when I dismantled my Hei, I found the vacuum modulator leaking and the bushings on the advance weights were gone. My milage has improved, how much I don't know yet. Idle is far smoother, I got it down to 500 rpm in gear. Running 8*. advance on idle, 37* at 3,000 rpm.
Brad, your 250 should be getting 15 to 19 on the highway. If you haven't upgraded your distributor, look into it. Iirc, the internal parts are the same as the sbc. You should be able to put in better weights and springs, probably even an adjustable modulator. Pertonix makes conversion box if you don't have an HEI. My 74 with the 250, 3 on tree, 3.73 gear and one bbl got 16, while my 80 van with same setup except had a progressive 2 bbl got 19. No they weren't fast, but they got me there. My best milage out of an inline was in my 62 Nova, 194 ci, one bbl and powerglide. That old gal would get 30+ on the highway running 80 mph.
