Tim is right about the procedure. Now is the time to also install new motor mounts and a new transmission mount. If you have a 4 speed your install is much, much simpler with the cab off.
I have just complete the reverse of what you are doing ... remove engine, tranny, rear end from a '51 chassis. Now have the frame about 1/2 done. Removed about 30 lbs of rust/grease so far.
Good Luck.
