I'm not sure I have the right understanding and I've done a search. Is it correct that the front subframe,brakes, and other front end suspension parts are interchangeable between 1st gen cameros and 67-74 Novas?

Yes they are. The camaro and nova share the exact same front suspension. I had a nova that got rear ended and i ended up buying a camaro shell. I salvaged the front end of the nova and it now happily resides in my 68 camaro. Everything just bolts on.

The 68-74 Novas will work. I don't think the 67 will.

Which Nova rear ends will swap in to a 68 Camaro. I have a chance to get a 10 bolt from a 73 Nova with 373 gears for $100. What type of modifications will I have to make so it'll work on my car?

none.

Sorry.I ment that rear will fit with no mods as long as you already have multi-leafs.