Ron, Funny! and Welcome! I want to do the exact opposite on my 69 Vert (drum brakes), swap out the XN rallys with some FC painted steel wheels
Here is a good read on Camaro wheels: Camaro Chassis - Suspension, Axle, Brakes, & Wheels
Your 67 could have come with 14x5 or 14x6 steel wheels or the same sized for Rallys (disc). The main difference is the rim contour. Disc rims have a different shape to clear the caliper then the drum version. So you can put a disc wheel on a drum car, but not a drum wheel on a disc car (Caliper will hit inside of rim).
1. All the "new" wheels are the disc profile, so not worries about using them on your drum 67.
2. The smaller flat cap with "inside" clips are designed to fit all 70's GM type rally wheel.
3,5. not sure what the backspace specs are, but the XG will work fine (68 Camaro disc 14x6)
4. Yes you can, see above
Not sure where you are, but if you are close to NJ, send me a PM