By 'vents' for the diffs, I presume you mean breathers, i.e. some way to get air in to a long hose going to the barb on the diff?
The plastic gizmos used by the factory are not available aftermarket, that I could find, nor from GM any longer ... but are easily found at the junkyard.
Alternately, I see lots of people use crankcase breathers, the little (2-3-4") air-filter looking things that you can get in a variety of colors and blinginess in the pimp-your-rice and 60's-musclecar-chrome-Mr-Gasket aisles of your local parts house.
Basically, you want something that will let air in and not let water in ... you could use a filter for an air compressor, or braze bungs into the bottom of your air cleaner, or whatever.
But a row of those crankcase breathers is just funny ... "You use 'em on your Honda, I use 'em for my rear end"
And as Dafey just said, yeah, the hose is your basic vacuum hose -- it's just air, so it needn't be fuel line or anything fancy.
Your other questions ... donno 'bout swimming caps for the dizzy.
My local hardware place carries rubber sheets in varying thicknesses, sold by the foot. IIRC they sell up to 3/8" or so thick, which would be enough to prevent a small third world country from entering your engine compartment.
I still have a piece I cut bits off now and again that I got on a junkyard tailgate as a liner; that one is fricken nuclear industrial or something, has bits of metal in it like it's made from recycled tires the way they do running tracks.
-- A
