What are the best small block to big block conversion mounts?

I used Schumacher mounts to locate a mid plate and front plate with my mock up 400 block and gutted 727. It did take some wiggling around even with the Schumacher mounts but it worked well.
Oh I just read your gts is a high dollar car, disregard the mid plate and front plate, however it's easy to remove brackets and welds if you plan to bodywork and restore the car