You can buy aftermarket trans mounts for GM automatic transmissions that bolt to the transmission bell housing bolts. These are designed for 55-57 Chevy's with the original front engine mounts to allow use of a later model automatic without a rear cross member. These new trans mounts bolt up to the 55-57 frame the same way the stock bell housing mounts did. It is my understanding that these mounts could be attached to the stock 46-48 Plymouth trans cross member but new holes would need to be drilled. Final result, front mounted Chevy motor, later automatic bolted at the bell housing and stock cross member without removing the stock cross member and weakening the structural integrity of the stock frame.