[FUNFER2]

Hello Paul & welcome.
The original rear end was a Salisbury IRS. I would build one with your plans and look at all of the Kirkham's you can to go by.

The frame is of course a round tube.

Engine either a 428 FE (in all street Cobras)
Or the 427 side oiler. You can buy a old block or complete engine or check out these new iron or aluminum blocks from Pond or Genesis.