Yes .5" TIRE clearance is fine but the minimum. What size tire are you going with?
Are you wanting a reversed lip design or standard lip wheel...Intro only offers up to 10" wide wheels in there reversed lip wheels, although for a ton more they will weld two hoops together to go wider. Were they possibly telling you the wheel only comes in a 10?
For what it's worth I have a 18x11.5 wheel and a 315/30/18 tire on my 68 w/ only a 2.5" mini-tub.
If you are stuck on Intro wheels give
HvyDeez a call, they are easier to deal with, they are who I used for my Intros.