I'll provide the other side of the conversation with the disclaimer that I may just be lucky and that may change tomorrow. When I installed a T5 in my near stock 1951, 3600 I fabed up a rear cross member and used a new S10 (rubber) trans mount.
So far, no problems in about 5,000 miles of driving and working the truck. That includes hauling 40 bales of square hay on the flatbed from the field to the barn (2,000 lbs. and 15 miles of rough country road). Hauling fire wood from the woods to the house. The truck has been on one longish trip of about 1,300 miles last year and will do that again in the next couple weeks.
Now, all the above said if I were doing the conversion now, I would not use a transmission crossmember for all the reasons mentioned above.
That S10 rubber mount pad looks to be very thick, and if allows a lot of axial movement, then I'd say that it should not be an issue for your T-5 in your particular truck. I wouldn't bother doing it, but I don't see your particular setup as being detrimental.
Carl
