I ran the one I had in the 48 for a long time with no crossmember at the back of the trans but don't know how wise it was. I'd think some support would reduce the stress on the ears of the trans though.
The Tri-Five Chevrolets were MADE this way; mounted at the front, and at the bellhousing, with the trans literally "hanging out there". Unless you're with Gas Monkey Garage, things should be just fine (see the episode with the yellow/black 55 Chevrolet hardtop). I have seen a LOT of aluminum cased Muncies with broken/repaired ears; whether they were broken when being used to cinch up the trans, or from "just hanging out there", who knows? Many guys have added a rear support crossmember just to be safe, or when converting to side engine mounts. The Muncie in my 56 Chevrolet Sedan Delivery is just "hanging out there". I am Butch/56sedandelivery.
The '55-57 Chevys came with front mounts and bellhousing mounts and no rear transmission support but they were mostly cast iron transmissions. Over the years I've seen many cars run successfully with bellhousing mounts but no trans mount but I too wondered how wise that was. My buddy broke a rear u-joint and the whipping driveshaft ripped the transmission (M21) from the bellhousing, leaving the four mounting lugs bolted to the bellhousing. Unless there's some thing major in the way, like the master cylinder/power booster, I would just add a crossmember to be sure. Especially if you tend to beat on your car.
I ran my '32 with a bellhousing mount and an aluminum Muncie hanging out in the breeze. Never had a problem and it sure makes pulling the trans easy.
... and it sure made stealing the trans out of a tri-five Chevy a 10 minute affair. Or so I was told.... I was a Ford guy by the way, so it wasn't me.... LOL.
Back in the day, saw a lot of Muncies with broken/missing/welded ears, pretty sure they didn't all reside a 55-57 Chev when they were broken. Probably more a case of treatment than application.
Chevrolet hung 'em out there when they were cast iron. A simple crossmember is cheap reinforcement. It stiffens the chassis as well as mounting the tail housing.
Chevy hung them out there- then thought of a better idea Chevy didn't have balancer bolts- then thought of a better idea. If one is to follow chevy's lead, then ,,,,
Cast iron is one thing, the big SM420s had no cross member. With an aluminum case, I'd run a crossmember, cheap insurance and a nice little project.
My '32 just had a Hurst front mount and the bell mounts up until about 1981 , running a 1961 T-10 with no ill effects. About that time I fabbed up a tail mount too , just for cheap insurance. Other than typical 2nd gear synchro troubles (unrelated) , no problems up to now.
My thoughts. An engine/trans mounted at 3 points will absorb some chassis flex. An engine trans mounted in 5 points may cause some stress at the bell housing/ trans mateing.
I figure on my flexie flyer partial boxed '32 frame, with all five being rubber mounts theirs plenty of twist to go around...
Usually any dirt track car with any aluminum trans with rear trans mount or one without it, after it hits the outside wall enough times will need the top left trans mount welded.
I would think that stock type mounts offer enough compliance for that not to be a problem. Now if he's using those stiff polyurethane mounts or a fabricated semi-rigid mount of some kind, that might be a problem. If you've ever bolted a crossmember to the stock Chevy trans mount and wiggled it - it's pretty flexible.
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