Sorry so late on a reply, just walked in the house and hadn't been in front of the computer all day minus the quick "Morning all" at login. Looks like a nice setup of a lot of different things, nice job.
The older one looks like an 88 or 89 according to the decals and straight front end. Toro took the 400 series and basically put a 20hp engine on it, the newer tower and dash, along with a few other cosmetic things. The 520HC ran those same 2 years then they went to making them all 520H tractors ( plus 200 of the 520-8 ) with swept front ends in 1990, and the decals went to the black + red scheme. They built 200 of the 520-8 for the 1990 year to see how they went over but they didn't sell so they never made them again. They are terribly hard to find, not many left out there but I just bought my 3rd one last week lol.
You now have more power than you could ever use on that one of yours.