long story

I've been putting together a 51 Chevy pickup, it has a 350c.i with a 350 trans. The motor mounts are solid the transmission has rubber, so I've been changing speedo gears, drive and driven, to test the speedo I had the rear jacked up and would put the truck in drive, the speedo would not work, although I could twist the sensor and get a reading, so I put it back tighter stuck it in drive and reeved it up pretty good, the speedo moved a little, but a cap flew off the drive shaft, it vibrated so I quickly shut it down. that vibe through the driveshaft cracked the bell housing clean in half. Of course this is going to cost $$$$ to fix now, my curiosity comes to play ? should I have used a solid transmission mount, and would that have been enough to save my tranny?

I don't think the mount would've made any difference.
Either the wrong fastener was on the clamp(s) that held the driveshaft in or it/they just weren't tight to start with.
Same thing with the bell housing, I've seen cars toss the rear of the driveshaft on the highway, the spinning shaft raises hell with the floor pan, but they don't break the bell housing, But I can see where it could if the fasteners weren't tightened correctly or enough.

fwiw, ymmv, mu point oh two.. etc. etc. etc.

the freakin slipped off when I installed the drive shaft I thought the cap wasn't properly seated so I smacked in and put the clip back on, a new u joint cost 9 bucks. I just wondered if I should run a solid trans mount when I reassemble everything.

Personally, I'd be more inclined to change all of the mounts out to new, and all matching material; for example: all poly mounts, or rubber if you wish, but new and matching.

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Agree with the Rrumbler. I'm pretty sure that Moroso states you must match your mounts, all rubber or all solid or all whatever.

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And I'll make it 3 who advise to use the same material in the mounts for all 3.

Have you driven anything with solid mounts? I never found it a pleasant experience... I would say leave the solid mounts for the race cars.

I heard this trick on the speedo cable a while back, sometimes the square tang on the cable slips inside the square hole in the speedo gear so its not always spinning at the same rpms do to the slippage, they say you can slide a small piece of electrical shrink wrap on the tang and shrink it so it fits in the gear a little tighter if you feel its too sloppy. Pretty cheap to try and you can always cut it off if needed. Best of luck