rear tire r100

I've had a 1978 R100/7 now for 2 years and need to replace the rear tire. Manual calls out a 4.00H18 size. I've also seen where manufacturers will list 120/90-18 or even 110/90-18. Note, I have the light alloy wheels vs spokes if that natters. The list of available tires that I've found so far...