Do you have a points distributor? Check your condenser and point gap, if you have pertronix check inside the distributor some times the two wire melt together, try jumping the ballast and see if that helps. I had a ballast that looked good but was faulty and head a bad connection.
With big bolt pattern spindles I run a 15x6 with 205 70 tire 4.5 back space (didn’t like that widened track width) and a rear 255 60 15 tire, when I got the car it had 15x7 with 4 in bs 225 60 15. I installed 235 60 15s it will fit but it’s tight.
To be honest I wouldn’t run a 60 series tire up...
Yep stock magnum heads with 340 manifolds had that combo for years then swapped to headers still using that same part number gasket with headers now.
Remflex Inc. 6004 Remflex Exhaust Header Gasket Sets | Summit Racing