68 Cuda grill studs............ Any tips ?

Sorry but no I don't. Too many years ago. For pop rivets, 1/8" steel easy pull would be a guess since that is what I find on my workbench today. The OEM welded in studs were replaced with generic "T" studs. #10-24 X 1". Cadmium plated. Stainless flat washers and nuts.
My 67 has the same "T" studs in its narrow upper and lower deck lid trim. Your 68 won't have the same example.