There should be a bunch of stuff in the archives about adding/changing
pop-tops.

Off the top of my head, don't get your heart set on one of the later tops. The
cut-out is very different than the one on your '73 (which has a one-year-only
cut-out, BTW). The '74 and later opening starts just behind the front seats (at
the "B" pillar) and runs almost to the rear of the bus. The "upstairs" bed
forms the ceiling of the bus in the rear and there is a
welded-in-at-the-factory kind of rib that runs from side to side in the middle
of the opening, above the rear seat.

That said, I've owned both types of pop-tops and I much prefer the later. The
bed in the later one is far more useful that the cot found in my '72. The
mechanism used to raise, lower and latch the later one is easier to use and is
also more secure when either closed or open. And besides, you get that "lovely"
plaid fabric on the bed......

Dave Kautz
'74 Westy