I've restored a couple Bigfeet, and it's a treat compared to most things. The only issues that unique are the foam, the spacebar (XT style), and if you are planning on getting it to breathe USB.
This is a nice guide for the spacebar (you might as well lighten the actuation of it while you have it out):
workshop-f7/model-f-improvement-dis-ass ... t6982.html
For the rust, just sand it really well down to bare metal (all visible RUST gone... okay if it looks "watermarked"), dry wipe well, then hit with an ETCHING metal primer. The cost for the etching variety is only slightly more, and is a considerably better move on bare metal. Then paint. I have only ever seen rust on top plates myself, but the bottom is nice and flat if wanting to do it too. Personally I like the look of two tone plates (top painted and bottom untouched).
If there are several of you actually trying to restore these, I would ask WCass (or maybe Muirium knows where to find) for the PDF of the barrel cutout, and have some place die cut several of them for one of you, and mail to each other. The cost of shipping foam is nothing, and could be in an envelope (it will straighten itself later when throwing the barrels in). Some place would do it cheap, turn out more consistent, and certainly save you time. Cutting 166 reasonably consistent holes takes a while (the most work of the whole project imo).
And then the USB... Khers' line sums up best...
Khers wrote: USB converting it was my first keyboard modification, easy peasy lemon squeezy.