Are you welding, or riveting in the new pan?
Unless you're welding around the ENTIRE structure, it ought
to be sealed. Auto-body seem sealer (available at most parts
stores) will work, but is a bit pricey. A good sillicon sealer will
probably work as well, for a few bucks less.
Let me offer

on the remainder of the installation,
based on my experience w/ the 90 Formula.
There is a VERY TOUGH gray primer on the undercarriage.
I personally would not sand-blast that away. And I hope you're
not considering sandblasting the interior, unless EVERY thing else
is removed, so that you can actually hose it out when you're done.
For coatings, I'm going with DupliColor spray on Truck Bed liner
on the underside, for sound/insulation, and rust-proofing.
This particular brand is durable, yet lightweight & easy to apply.
The super tough coatings like POR-15, Rust-Bullet, etc., are excellent
products, but require good surface prep, are kinda pricey, and have
no insulating properties. Inside, underneath upholstery, I would think
something like RustOleum would be sufficient, which is what I did
on another restoration project.
Good luck.