PLEASE HELP Busting Transmission Housings!!!!!!

[VJJ]

I have a 71 CHvey truck short wheel base with a single peice driveshaft
I've busted 2 transmission housing I no its a vibration or somthing is out of line but i've check everthing this is what i've check so far. I changed the driveshaft (check and made sure the weights was still on) i replace both ujoints, I'm using stock motor mounts and transmission mounts made sure the driveshaft was running straight throw the hole(in the crossmember) but i've broken another transmission. It doesn't vibrate when driven normal just when i get on it in high gear roughly 65-70 mph just start to shacking like hell and snap the transmission.

The first time it broke around the bell housing
the second time it broke around the bell housing and in back of the transmission also slammed the drive shaft against the crossmember and broke it also>

This is what i've changed:
1. Its a very healthy sbc i raised the motor aabout 1/4 of a inch to get header clearance also raised the trans.
I lowerd the back 6 inches
HAVE ANYONE HAD THIS PROBLEM IF SO PLEASE LET ME NO WHAT I NEED TO CHANGE OR CHECK