Drag City Casting on FB casts the Pontiac rear end without the brackets, and for less than the full kit. I've bought from him and he's reliable and has good products.
https://www.facebook.com/dragcitycasting/?fref=gs&dti=173972342685783&hc_location=group_dialog
https://public.fotki.com/drasticplasticsmcc/member_dealer_directory/drag-city-casting-/parts/dsc08487.html
VCG Resins has a drag bell housing that you might be able to use. It's under Drivetrain almost halfway down the page. He also has a bellhousing w/mounting plate that can be tweaked to add those extra mounts.
http://vcgresins.com/main.html
Sounds like an interesting project. Good luck.