Does anyone know if you can put a 3 speed tranny, or even a newer 4 speed overdrive in a 62 Invicta? This car drives great, but it runs way to many RPM down the highway at 75. The 401 has plenty of Low End Torque, so it could handle an overdrive with no problem.

Will a BOP bell housing tranny say a 400 turbo, or a 200-R4 with the universal bell housing fit? If not, what are the issues that keep it from working, and can they be worked around.

Unless you want to try and find an adapter, you are stuck with the Dynaflow. If the engine RPM is more than you like, you might find a higher ratio rear axle.

You can put a superturbine 400 after the 401 from 59 and forward if you want to. It has been discussed previously in the forum many times. try to look up an old topic.
Also my experience is this:
You need a flexplate and starter from a newer buick that goes with it. Years 1964 to 1966.You would have to make the hole in the flexplate slightly bigger to fit the crank of the older engine. also probably get a longer piece of the drive axis, yoke and new mounts for the sp400.
also balancing the drive axle.(excuse my english).
I think that the dynaflow is longer than the SP400`s wich is L=appr.70cm.
Finaly, maybe its better to keep the dynaflow, I have heard that it is a good transmission, maybe you can change the ratio some easier way.
good luck rob