Poor mans Hemi car. (M-body)

That's gonna be one sick Dip when you get it all done. As heavy as F-M-J body cars are, I'd be doing a little creative metal removal in non-structural areas, and where it won't show. Like behind the bumpers, inside the door bars, etc. But that's just me...

Speaking of lighter K frames, sometime in the last century (around 1980) Paul Rossi built a Mirada for either Super Stock or Super Gas (9.90 et) racing. I think it was covered in Hot Rod, or maybe Car Craft. Anyway, to get the weight out and get rid of the "odd" transverse T-bars, they grafted an A body K-frame and torsion bars into the Mirada.
Too much work for a budget build, but there ya are.
I saw a guy at a show with a 440 in a Volare or Aspen, he said he'd used un-modified E body headers in his application. Might be a good tip for anybody considering a B/RB in one of these chassis.
Good idea on the guages, Stewart-Warner has some retro styled guages that would look good too, if that helps any. Good luck with the project, it sure will be cool and unique!

Alan