I have the 69 bay also, mine has the headbanger shelf and I’ve always thought it was a bit of an odd finish to the inside. I’m now thinking it’s been retro fitted by a previous owner
Same situation, old and new metal to protect,I can only say what I did.
I bought a set from Frost’s, once all the gunk was removed from the underside there was a degreaser, a metal prep and then the final coat
https://www.frost.co.uk/brands/por-15-rust-prevention-paints-coatings/?psafe_param=1&gad_source=1&gclid=Cj0KCQiAz8GuBhCxARIsAOpzk8wzar-VjUqJ6h6w3yDiYrObpHw6F2CSf6v-e2QcIPLRHquH_81KHNoaAkA0EALw_wcB
I used POR 15 on the underside of my bus, lots of prep work but the finish and protection is...