I'd say 275/50/15, but there'd be lots of tire bulge.
If you're gonna put big tires on the stock rims, you should probably stay around 245/60/15 or 245/50/15 (if they make that).
If you get 15x8" rims or anything bigger(preferably a little wider than 8, like 8.5 or 9"), you can get the BFG 275/50/15s (for back).
-----------------------------------------
'79 Mustang Coup w/ 4" Cowl
Stock 5.8L, C4 w/ shift kit
Holley 750 cfm, Edelbrock Performer RPM intake
1 5/8" MAC shorty headers, Al driveshaft
2.5" Off road H-pipe, 2-chamber Flowmasters
Front: 225/60/15, Rear: 255/60/15 Eagle GT II
Weld wheels (15x6;15x8), 8.8" Rear w/ 3.55s
14 x 4” K&N air filter (getting the Xtreme setup someday)
"Red, thou art my companion. Hasten now your quickened metamorphosis to
Green that I may conquer all who dare abide there beside me. May they be left
thither behind burnt black." ---Fox Body