[Captainfab]

You do not need any aftermarket conversion mounts, unless you just feel the need to spend money on something. The original style engine mounts in your '65 C20 will bolt right up to the L31 Vortec 350.....it is still a GEN I small block. If you want a better engine mount, you can convert to the '73+ engine mounts with a set of frame brackets from a '73-'87 C20 or C30. The clamshell on the Vortec 350 and the engine mounts in the Escalade will work with those brackets. If you do go with the 4L60E or 4L80E trans, then you will need to remove the bellhousing crossmember and install a trans crossmember. For that you can simply use an auto trans crossmember from a '63-'87 truck, Suburban or 2wd K5 Blazer.

For the PCM reflash you will need to send your PCM into one of many vendors that perform such services. I do not have any experience to be able to make a recommendation.