My guess is your Chevy II would be 1.5 to 2.0 seconds slower than the Mustang. I am thinking the Mustang should run high 13s or very low 14s and your car likely will run 15s to 16s.For a comparison the 66 I had ran 14:30s and likely would have been a couple of tenths quicker with tuning at close to sea level. It was a normally aspirated 292 with ~ 250-275 hp, Saginaw 4 speed, 4:10 gears and drag radials.
If you can afford and want to do it, an OD 5 speed and 4:10 gears would be a help in the quarter and still be a great street car. You will need a posi or locking rear and if you get it to hook-up, new rear axle shafts.
I will guarantee you will get alot more looks and positive comments with the 250 than a sbc or the Mustang and a quick six definitely messes up people's minds. Just remember to have fun with it.
