Another crossmember, if mounts are to ridged, can cause broken bell housing too. On heavy duty trucks, if they have a rear transmission mount, it's usually a leaf spring type deal just to share some of the weight but not to locate or restrict movement.
Again, what engine mount arrangement do you already have?
I wonder if you have a cast iron housing if it'd be just fine, and if it's a common housing might just try it and Inspect every now and then, replace and figure out plan B if ever an issue. What is it, 7" thick? Not a lot more leverage on the bellhousing which might have accepted a longer heavier 5 speed already without issue.
