On a 1975 Sporty, you're going to be limited with options due to age and frame style. Wider tires are hard to fit due to narrow fender mounting/clearence. I know guys that spaced the fender rails out, and used a wider rear fender, cut to fit, to make it work, however, the chain will limit your tire width, as with a drum rear brake, you can't shim anything out. Not sure if you can upgrade to a square swingarm and rear disc brakes.
The steering stem diameter for your bike is 7/8", and not the 1" like the newer harleys. You can get conversion cups to make a wideglide fork fit here:
The other alternative would be to look for an old Sevi-Car springer fork, (OEM) which had a 7/8" stem.
You can also look at WG conversion kits, but your fork legs are 35mm, and not 39mm, as the kits are made for. The 39mm (1987 and newer) fork legs are relatively cheap, as well if you get them used and want to go that route, as the 35mm legs look kinda skinny in a wideglide set up....