I would think twice (and more!) before installing a roof A/C on a 1974 Trillium. I have a 74 Trillium and have to reinforce the roof right in the center as the roof sags in spring when snow starts melting. That's nothing compared to the weight
of a roof A/C.... Considerable reinforcement would be needed and the total bill (including A/C would at least be in the 2,000 CDN$.
Someone can install a 5,000 BTU unit for 135 CDN$ (85$ for the A/C and 50$ for lintake and exhaust material) !!!!
Ok, you are going to loose may be 3 cubic feet of storage but there are so many ways to recuperate that storage lost... More than that your are going to increase your gas costs for ever by adding that friction and dragging factor on the roof. Internal installation adds only let's say 25 pouds (may be less) no drag factor and perfect balance as it is just over the wheels...