I also copied Doug's car and used 2x3 between the frame connectors (welded through floor) and rocker box. I had a really hard time welding to the rocker box due to contamination in the box itself. Not sure how to get in there and clean the backside of the box where the 2x3 is welded.

After I hole sawed through the top of the outrigger to locate the main hoop. This allowed me a solid fore-aft measurement for the roof halo, which works good in a 66-67 b body IMO. Once you have the roof halo leveled and tacked in you can raise the halo/main hoop up and slide a place under it to get the a-pillar bar measurements. I am 50/50 at this point between plates and outriggers on to land the forward bars after all of the trouble I had welding to the rocker box. At lease I can clean both sides of the floor pan for the plate.

That said if I was starting from scratch and not as a broke 19yo I'd have the car blasted before doing anything. The latest rendition of my red Charger has been the slowest going because the car is painted. If you google the NHRA rulebook 2017 is puplished and for what you are looking at it's the same as my 2018 book.

hth