Agree 250 Sonnar is border line hand holdable, and that is without 2x Mutar.
Why the 350 Tele-Tessar never had a tripod collar is beyond my comprehension. I have an off-brand collar, but as lens was never designed to accept one, it cannot be mounted in a correct position. Then add non-IF design, lens extends by about 3/4 inch, and things get worse. The lens mounted directly to body and rested on body's foot plate is so front heavy, I'm worried about distorting the mount or body itself.
Tele-APOTessar 500 CF has a foot plate and with that it is so much better in use than the 350.
The Variogon 140-280 is just as big and as heavy as the 500, its first C version with Compur shutter had no tripod mount, but Hasselblad made a plate with thin metal band to go around the barrel. It's OK, better than nothing, but I was never in agreement with how narrow that band was and potential stress it would exert on the barrel. So finally have found the CF version with more proper collar by design and that is much much better.
But the 350/500 are great to shoot with. None for sports or action shots due to long throw (slow) focusing (absolutely forget zone focusing), but for right reasons that's where that long throw shines with very precise focus.