We were attempting to put a 360 into a 318 70 Duster. The 318 PS mount worked but the DS was like yours. DS mounts are near unobtanium status.

I ended up calling Schumacher and they sold me just the DS mount. About $70. It appears that they may be out of stock, I'd give them a call.

Schumacher

1972 Cuda 340 4 speed, 2001 Ram CTD 4x4 6 speed, 1970 Duster 408 4 speed, 1996 Ram 5.9 2x4 auto, 1965 Coronet 500