Bill.
I don't think letting that mount float is not a good idea. The rear transmission mount actually locates that engine in the fore and aft direction. If it is allowed to move around or tries to rotate it might work your front mounts pretty good. By making that cross member rigid it will add a lot of torsional stiffness to your frame. If you want to make it removable then you might consider adding a threaded doubler to that cross member flange. Make it 1/4" thick and tack it to your frame. Then run the tap into the side of the frame rail. That should give you 7/16" thread engagement. Tap it for a fine thread and you will increase the thread count engaged. Then all you have to do is put a fillet weld around your 1'4" plate to the frame rail. Might find a school shop program that will do it on the fly. Buy them a beer ...opps! make that a root beer and call it a day. Good Luck!
George
The motor mount system I'll be using should be more than sufficient to keep my little V6 in it's place...it's designed for an off-road racing buggy with an engine nearly double the weight and 4 or more times the horsepower and torque. If I do see signs of excess movement, I'll put bolts through the cups and tube once I have the angles where I want them.
Before doing a doubler and tapping, I'd probably just go with what I originally had planned...welded in crush prevention tubes and bolts running clear through the rails.
I wish the shop program here would take in outside work. Between the T project, the Cougar project, the trailer project, etc; I'd keep them busy for several years. LOL