Doesn't the Chry V10 have the LA bellhousing bolt pattern on the block, since that's the basic architecture it was expanded from?
A stroker 408 LA motor would fit better in the Imperial, with a modern self-learn EFI on it, too. Or even a salvage yard 360 EFI motor, for that matter. Most of the Imperial's EFI is in on the air cleaner base "plate", as I recall. If it's running flakily, it might just need the band clamp on the air cleaner secured with NO air leaks around the edge of the air cleaner . . . as simple as that might sound, the MAF is in the air cleaner snorkle. Remove the air cleaner top with the engine running, loss of air flow signal kills the engine.
Just some thoughts,
CBODY67