My 306, with a shortblock assembled in 1987, runs 12.90's on the motor, in my 82, carbed, thru a C4 and 4.10 gears on Drag Radials.
Its got a 30 over 68 302 block, a salvage yard 78 Thunderbird 302 crank and rods, and forged TRW pistons, Speed Pro rings and bearings. It still has the Rhodes lifters 270 deg/464 lift cam and lifters that I installed in 87. Ford SVO timing chain. It currently has an Edelbrock Performer RPM, Holley 600 (80457), and some 94 Lightning iron Gt40's, with a Comp spring package. With a mechanical fuel pump and stock tank, e fan, and MSD Digital 6. Also has 1986 Blackjack semi longtubes and dumped Super 40's.
It's not balanced, but it will spin to 6k with the stock stamped rockers! Has in excess of 95k miles on it.
Kind of amazed that it runs this well!