-- Tried to install a 426 Hemi flexplate, but could not mate the 318 torque converter to the 426 Hemi flexplate without using approximately 1/2" spacers in between the flexplate and the torque converter lugs. I felt that this would not stay together so opted to re-drill the 6 bolt hole flexplate (318) to the 8 bolt hole pattern as a temporary measure to see if it would work until the correct torque converter for the 426 flexplate could be located . Believe it or not, this set up worked fine for the 8 years the truck was driven.
-- To make the bellhousing adaptor plate I used an old 331 Industrial engine block, A-727 transmission case and the original 1956 Chrysler spacer plate(between engine and transmission) for patterns. The 727 case bolt pattern is the same as the 331 and 354 for 3 or 4 of the bolt holes. An aluminum plate was drilled to this pattern for the 3 or 4 bolt holes that match, using the 1956 Chrysler spacer plate as a pattern. The 727 case was used as a pattern to locate the dowel pin holes for the adaptor plate by bolting the aluminum plate to the 727 case and using centering pins in the 727 case dowel pin holes. The remaining holes in the new adaptor plate were located using the threaded part of bellhousing bolts that were machined with a center point on them. Adaptor plate was counter bored on one hole (next to RH dowel pin hole) and a thin head bolt machined to fit (in counter bore hole) flush with the face of adaptor plate - this bolt secured the adaptor plate to the engine block. As I recall there are two threaded holes tapped in the adaptor plate on both L & R sides, near the dowel pin hole locations, to secure transmission case and/or engine block to adaptor plate. The thickness of the adaptor plate that worked with the stock 318 flexplate and torque converter is 1/2". After aluminum adaptor plate looked like it would work out ok, the final adaptor plate was made out of mild steel plate. The adaptor plate probably should have been 5/8" thick which would allow for more clearance between the torque converter and flexplate when bolting or unbolting the flexplate to the torque converter.
I hope this information may be of some help.