flywheel and starter
The 390 flywheel ring is .009 inch smaller than the 460. The pitch diameter is the same. That means the teeth are exactly the same profile. What does change is the number of teeth on the ring. If you use a 390 ring you will not totally engage the teeth properly with a 460 starter. In other words, the starter will be too far away by about .005 inch. [1/2 the .009]
Will it work? Most likely. Will it work exactly right? Never. I had Kentucky Clutch balance a flywheel for me recently. I had a 1986 ext balanced flywheel that I needed to make into a neutral balance to fit a 1977 internal balance 460 that was going to replace the 1986. I had a machine shop turn the trapezoid lug off of the back of the 1986 wheel. KY Clutch then only had to drill a couple of holes to balance the wheel to 1/4 oz. If you need this kind of help, it is hard to beat this shop, their price, or their knowledge.