I know it's not what you asked for, but why not put a regular small bodied distributor back in? Seems like you're trying to re-engineer the car to accommodate a GM part.
There's a web a site where the guy fabricates a bracket that's secured to the underside of a stock Mopar LA electronic distributor. An HEI module is secured to the underside of the bracket. Pretty much out of sight and does the same thing as those HEI distributors. I thought I had bookmnarked that site but apparently not.
