Going down the road in a straight line, you probably won't notice any difference. Where the problems may occur is in corners when you are applying a side load to the wheels. Like I said, alloy and steel rims flex differently. One, most likely the alloy, will "give" or flex a bit more. That may throw off the wheel alignment and possibly induce a wobble. But, maybe things would be just fine. I can't speak from experience because this is something I never tried. This warning against mis-matching rim types has been around for years. There must be good reason for it and I never felt the need to test the theory.
Yes, you would need to swap the discs but it's a simple bolt-on swap. The 2 piece style rotor on that '75 wheel has a different off-set compared to your '81 disc. It won't align with the caliper. The only caution you'll need to take is with the length of the disc mounting bolts. If you look at the second last pic for that '75 wheel, you'll see that the disc mounting holes are "blind", they don't go all the way through the hub. Look at your current '81 wheel and you'll see they do. If you use a mounting bolt that is too long on the '75 hub, it can break a chunk out of the back of the hub. Compare the mounting bolt lengths between the two wheels. If the '75 ones are shorter, use them.
