I'd strongly consider getting rid of those old fixtures. You can go to HF, L's, HD, other big box stores, or Amazon and find reasonably priced LED fixtures w/o any ballasts. Not having to deal with replacing ballasts is a plus in my book. You have to check the package to see if the fixtures are dimmable or not. There are now quite a few on the market that are dimmable.
I went with track lighting some time ago, when LEDs were just starting to come onto the scene, so it has been a learning curve for me. This is especially true with lamp selection. Read the package to determine the actual wattage and lumin output. Unfortunately, I have quite a mix! That's something I need to fix. GRJ is quite correct when he says you can only put 7 of the 20 watt LEDs on a dimmer. In his case, he has enough ceiling height and the right type of LED that he can get by with fewer LEDs. My ceiling is lower, and as I alluded to above, some of my LEDs are not the best, to say the least! (Low lumin output). So I ended up needing more fixtures to cover the area.
Chris
P&O