Hafthorr,
I have a non-flared Speedster, probably about the same vintage as yours.
I tried fitting 7" wide Fuchs but they were too wide for the rear.
7" will clear on the front of the car but they'll rub the body by about an inch in the rear.
Mine is an IRS drum brake VW based chassis. I have discs in the front. I ended up with 16"x6" wide Fuchs all the way around. They "just" clear in the rear.
If your wheel/tire combo has different clearance side to side, that's another problem.
My Speedster ended up with the same tire clearance on each side, so I didn't have any problems with that.
Greg