50 or 60 series tires?

I am about to pull the trigger on new tires for the 67 DART.
I have installed a offset kit from Dr Diff and have purchased a set of 15 x 8 4.25 backspace
Cragar rims.

I am going with a 255 tire as the 275 is a wee bit to close on the sidewalls.

For those of you who are running a 255 or close to it, are you using a 60 series or 50 series tire?

Thanks