73' B3 340 Duster pro touring build

Those are 17s right? Would you be going to 18 or staying 17?

Yes, the current wheels are 17"X11" rear and 17"X7" front...the new wheels will be 18"X12" rear and 18"X7" front. I had considered 20"X12" for the rears so I could run a 29" tall x15" wide Mickey Thompson SR Radial which is about the same dimensions as my M&H Racemaster's 390/40R17's. Unfortunately, there isn't a 18" tire out there that is the same size as my 390 Racemaster's, so I'll be giving up about an inch in tire width and height. My current tire is 29.5" tall by 14.3" section width.