As a basis for comparison, had a buddy with a 69 Chevelle, with a factory 427/425 hp, 4 speed, 4.10 gears. Through the mufflers on 9 inch slicks he ran 13.0s. Similar car, '70 Chevelle 454/450hp, 4 speed, 4.10 gears, open 2" headers, and slicks ran 12.50s. Both of these are at 1400 feet elevation. I don't know for sure how your engine compares to these, power wise, but maybe that gives a basis for comparison.

Pat