I too will be doing some custom work to a 1974 70. The spindles on the front axle of a 70 will accept the round fender 3/4" spindles. I wouldn't use bigger wheels on the original 70 spindles, as they would be too weak. The round fender spindles are heavier steel and will hold up much better. This way the front or narrow front round fender wheels will mount up with no problems. As for the rears, I'm lucky enough to have a set of the John Deere five lug hubs that were made to mount onto the '63 & '64 110's with smooth axles. The keyway is already notched into the hubs, and matches the keyway cut into the 70 axles. Now I can go with a 12" wheel. Only thing going with a larger diameter wheel is, it really cuts down on the fender/tire clearance. To get over this, you'll have to shim up the fenders with flat washers, between the transmission and the frame. Not between the frame and the fenders!
Good luck to you, and post pictures as you go!