I've done it both ways with my T5 behind a 235. With the front engine mount and the two bell housing mounts I had some mixed feelings about bolting down the tailshaft of the aluminum-cased T5. With almost 5 feet of frame between the front and rear mounts I had concerns about frame flex placing an undue load on the transmission tail-shaft housing. Because of this, I ran the first 3K miles without the cross member with no evident ill effects.
Recently, I added my own cross made out of 1/2X2" flat stock with angles that let me add a rear mount to the flat S10 pad. However, I added some additional space under the rubber mount for some rubber pads and don't really torque down these rear mount bolts. If you have a brake booster, you may run into some additional clearance issues. The flat stock mounts on the bottom frame rail with a couple 1/2" bolts so dropping it is quick and easy and it gives me the visual effect that the rear is supported. With the heavy sm420 hanging off the bellhousing with no additional rear support, I wasn't really all that concerned.
In my opinion,and after running it both ways, I'd say that if it makes you feel better go for it. If you have booster/master clearance issues I really don't think that on a street machine driven under normal conditions, it wouldn't hurt to run without.
How's that for an ambiguous answer?
Dave
