302 .030 over, 9.5:1, small port iron GT-40 heads with less than 200 CFM intake flow, dual plane air gap intake, 600 CFM double pumper, 226/230 Crane 110 LC hydro flat cam. 4.56 gear, 4500 stall converter, 28" tire, car weighs 3250 with driver. Runs 8.00 1/8 mile in Dallas (about 750 MSL). That's about 12.5 in the quarter. And the car is way bigger than a 79 Mustang.
Put a 347 under those heads, make the compression around 10:1 - probably custom pistons for that - and run a 230 something intake cam. Pay careful attention to all the details - deck, piston-to-wall, bearing clearances, bore finish, quality of parts, etc. IMHO - If you can't make that go mid 12s in that Mustang, you've got too little converter or a messed up suspension. Or a dead elephant in the trunk.
1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
Owner built, owner abused.