I installed the SPAL kit, P/N 3300-0030 in my '73, and it works great. It may be a bit slower than original equip, but it is much faster, quiter, and smoother than the one a friend of mine used. He took the cheaper route with a less expensive model that was ok for flat windows. I had to use the dremel tool to rework (minor) the slots on the door crank spindle to get an adaptor to fit, but that may have just been mine. Conversion wasn’t hard, but with a lot interruptions it took me a weekend from start to finish. Without interruptions, I would guess maybe 4 to 6 hrs.
Which ever one you go with, be sure to get the heavy duty motor. The smaller less expensive units are for flat windows. The curved windows require more power.
-Jim