I just purchased some old style alloy rims for my 76 280S. I know they came stock with 185's on the steel rims, but I'd like to go with a bigger tire. What are your recommendations?

DEPENDS ON THE SIZE OF RIMS THAT YOU HAVE FOUND. IF THE RIMS ARE 6 X 14 YOU CAN USE 205 70 X 14 TIRES & SPEEDO WILL BE WITHIN
1 OR 2 MPH AT HWY. SPEEDS. MAKE SURE TO USE SPEED RATED TIRES DESIGNED FOR MAX. SPEED YOU PLAN TO GO.