Hello,

my father has a imperial with a 360 and a 904 in it.
He bought the car without drivetrain and this combination laying around.
Since winter there is a problem with the car.
If the engine is warm and you put it into gear, the rpm goes way down. Often to the point it just stops (if you put it in reverse, always).
If you hit the gas before it dies off, the car basically goes off like roadrunner, with the engine shaking.

Is the converter locking up or something?

 

This is quite the coincidence a member on a Ford forum with a mopar powered postal van was talking about this same problem, putting the transmission in reverse and the motor shuts off.
About the only thing I can think of is a wiring problem that happens when the shift linkage or the motor/trans shifts a little from the gear change. I would check all wiring and connections first.