The 318 mounts should work. Specifically what problems are you running into (pictures would help)? Are using the 57-58 392 center sump pan (it's the only one that will fit the body)?

1957 Plymouth (Hemi, Dual Quads, A833 4 Speed 9 1/4 w 4.10) Sold
1937 Dodge Pickup (Hemi, 6X2 intake, 46RH, Dana 60 w 4.56) Sold
1968 Plymouth Valiant 2dr sedan (354 HEMI, 46RH w/4.30 gears) under construction