Engine Replacement / Keeping Transmission

[Spline64]

Hello all,

I've got a 1973 CMG C2500 long bed 2wd that had it's original 454 replaced with a 350 years ago that has since "retired".
I have finally got another 454 to replace the 350 in hopes of one day restoring this truck to it's former glory for my son.
The original TH400 Transmission is still installed and in good condition. I have heard that I can easily ruin this transmission unless when unbolting the 350 from the flex plate I push the torque converter back into the transmission. I would also need to support the transmission so it would not move when the engine was disengaged.

Is it just that simple or is there a better way to leave the transmission mounted? Or can you even do that?

thanks everyone,
Pat