My '74 project XLCH came with a Lester 16" rear and front forks/twin discs/Morris 7 spoke off a period Low Rider. The 16" rear is too fat and barely clears at many points; I've had no luck finding an 18" Morris so have snagged an 18" Borrani wheel in real nice shape. I see in the parts book there is a twin disc compatible "aluminium" front wheel for 1978 and later with a -77 part suffix.
I presume this wheel was first used on the 1977 Super Glide?
A 18 inch rear tire will be taller which might cause rubbing if the shocks are too short.If the tire is fat maybe consider a 120/18 (thinner).Salty
The 18" was stock; as are my shocks so there won't be an issue with height.
I'll use a stock 4.25" X 18" tire, too. I need to get the right Boranni front wheel to keep the dual discs.