What about the JVXRACING.COM Adapter plate.
If I read correctly, they make a thin adapter plate to bolt a A518 directly to a Big Block with the required spacers to keep the Starter, Converter and Ring Gear all in the correct location?
P.S. John Vinson will answer any questions you have.
His adapter sells for $675. His Heavy Duty In-house Built Transmission for $2200. and the Base lock-up converters (aprox 2800rpm stall) start at $450. +
Quite a bit of mods required to the trans cross member area and of-course shorten the drive shaft.