what car? pretty easy to make a high 12 second b-body with a 440. My old 67 coronet weighed 3850 and ran 12.90 @ 104 with a 9:1 .030 over 440, cast junk pistons, stock rods, crank, stock 906 heads, stock valve trane, Summit 488 cam, RPM intake Holley 750Dp, Cheap headers, Mopar 175k converter (2600), MP shift kit, and 3.91's.
You can do it with 383's and SB's but it will cost you much more.
A 10:1 440, with alum heads or ported iron heads, 500+ lift cam, headers, RPM intake 750-850DP, 2800 stall and 3.91's will get a 3800lb B-body into the mid 12's easy. In an A-body you'll be in the 11's.