With the '90 roller block you described, even if it doesn't have a roller cam as original equipment, you can still upgrade it with one. From that poing you just need to use the HO (351W) firing order. So pretty much any of the mild Mustang grinds will work. Don't get too crazy though, since piston to valve clearance might be an issue with the stock cast pistons. Oh, and make sure you've got the correct distributor gear, or you'll chew it to pieces.