Fyi, Pontiac used front engine mounts up thru 58. My uncles 58 Tri-power Superchief hasn't had any problems with his front mount.
I made side engine mounts for my 55 using 2" square tubing and some 1/4" plate . It's tricky on the drivers side because of where the mount need to be because of the gearbox is on the frame at that point.
The 700r4 should fit, it's about the same length as a short tail th-350. With that much hp, you may start looking into a differant rearend also. 49-56 rearends don't have a posi and come with 10 spline axles and arent made to hold to a strong engine.
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Illinois Outlaw Gassers
6.27@107
9.97@131
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