Before I installed my new crossmember back in 1981, I attempted to seal the metal in the interior side of the unit by dumping a can of marine primer thru it. After that dried, I tried spraying a gloss black overcoat inside there as best I could. After primer and painting the outside with Rust-oleum black, I smashed a glob of "duxseal" over the two weepholes on the top plate to keep water from running thru there. I avoided painting about an inch from the edges prior to installing for my welding bead.
Here more than 30 years later, my unit is still solid even though it has been on the road nearly every day since then.
If I had to do it over again, I would use self etching primer on the inside, then overcoat that as well as doing same on the outside.
One thing I haven't heard anyone mention is that if you do use a factory duplicate part like I did, they come with the "tabs" at the edges just like the original part had.
When removing your old part it is necessary to un-bend those tabs on the old part to pull it out of the frame. To accomplish this, it is necessary to put a straight jack behind the crossmember to spread the frame side rails apart about an inch. Leave that jack in to facilitate installing your new crossmember and guide the "tabs" into the slots in the frame side rails on both sides as you loosen your jack. One the jack is remove, you can now use a hammer to bend those tabs over flat against the frame rail. Now you are ready to weld.
Don't forget that it is necessary to remove the radiator and it's support channel to complete this operation, since the support channel has studs that will interfere with sliding the new crossmember into the frame.