The space bar on the 122-key terminal is standard, same size and shape as the M but with a thinner wire.
Yeah, I'm glad about the F122 space bar, but replacing the foam on one looks like it would be a chore. That thing is such a beast!
Back plate needs to be separated right?
I'm afraid of separating the plates as the foam is starting to disintegrate...
I also saw somewhere said that you need some clamps to hold them together while assembling.
It sounds like a tough process, I'm surprised you can do it in 30minutes. 
Yes, unfortunately, whenever replacing foam on a Model F, the keycaps need to come off, the plates need to be separated, the barrels/flippers are removed, etc. If you use a plastic spudger, you can slide under each barrel and lift them out pretty quickly, setting them in order on your work space, with the flippers still in them. They go back in really quickly that way also - two at a time if you want to use both hands and have some manual dexterity, lol. I think it took maybe 5 minutes to pull all the barrels and flippers in the AT I was working on the other day, and a little less time to put them back in.
For the XT/AT back plate removal, just get 5 or 6 little C-clamps and protect the clamp surfaces with electrical tape so you don't scratch up the top/bottom plates. You don't need much pressure at all on the clamps. After clamping around the edges, bend the single locking tab back a little so that the top plate will be free to slide out of the holding tabs. Put the keyboard on end with the top plate in position to slide down, then placing a wooden block on the end of the top plate, give it a few good taps and it will move down and be released from the holding tabs. From there, it's easy to lay it down and carefully support the assembly with a thick book on each end (or a nice wooden jig as some folks have). Now remove the clamps, and lift off the back plate. If you are just adjusting the space bar, you can leave the barrels and everything in place, being careful not to disturb anything, and just use a thin tool like a credit card or spudger to hold the space bar flipper in place initially while installing the back plate.
I made my own replacement foam out of kitchen shelf liner and a 1/2" hole saw bit that I sharpened into a hollow punch on a grinder, lol. I think the shelf liner was about 1mm thick or so. If I were to do it over again, I would buy a cheap punch set at Harbor Freight and use a slightly smaller punch so that the barrels fit more snugly in the holes to begin with.
It could be that my AT had a poorly bent top plate, but no matter what I did, a single or double layer of foam over the entire board didn't make the center rows of barrels as tight as the top and bottom. Even with two layers of foam, the top and bottom rows would be nice and tight, but the rows to the center would be slightly loose. Looking at the side profile of the top plate attached to the bottom plate, it was obvious on that the gap increased toward the center of the board, being the biggest right about where the center row of keys would lie.
To combat this, I put a single layer of shelf liner over the entire board, another layer over just the middle three rows, and then a third layer over the center row only. It worked beautifully and the entire keyboard has a nice solid barrels with nice tight keys. The pitch is now the same from the top of the keyboard to the bottom as well.
In addition, building up layers on only those rows that needed it made reassembly easier. In fact it was much easier than when I had tried just two layers on the entire board. This makes sense due to a smaller gap at the top and bottom, so adding thickness there was counterproductive and only made it harder to clamp and reassemble.
There are already numerous tutorials and threads about how to work on the F, but that's how I've been doing it thus far. Your mileage may vary.
