Hi Arthur.
SST does make the Perfect fit kit for the 400. The transmission mounts in the same location whether its big block or small block. I chose to build my own because I wanted to set my elevation of the transmission and add the support I cut out. My tail shaft of the TKX is in the same position as the 833.
The US Car Tool sub frame connectors and torque boxes are a nice upgrade regardless. I have their torque boxes on my car.
SST and American powertrain kits only have the transmission mount for the TKX. Since their mount is stronger than OEM I believe they feel it is not necessary? Many people are running their cars that this way and seem to be fine. There is more space with nothing there for sure.
The way the US car tool subframe connectors tie in would help in not having the crossmember support.
Soo. I would prefer the support over the top as that's what I did. But I think you could get away without the upper support.
I think the subframe connectors and torque boxes are harder to do than how I did my crossmember. If I was to do it over again I may not have leaned it back. The sheet metal fit better but the inside gussets did not fit as nice. The main welds are flat and on heavier metal.