Shoud I gunkote the rec. before I rivet the trunions in, so that the entire inside has a layer of rust protection?  

no

I don't think it's a bad idea to apply a little gunkote to the inside of the receiver before you rivet.  Especially to the areas that'll be covered up by the trunions or areas that'll be difficult to reach with an airbrush once it's assembled.  It's probably a little over kill but I do it that way. I also spray the outside of the trunions before I rivet them too.  I don't do the outside of the receiver until I'm finished riveting though.

If you use a zinc phosphate primer it will usually squeeze between most of the joints and give it a corrosion protection.  The paint will not, obvioulsy.

That's why i daub on zinc phospate, instead of spray. ideally, you'd do a full immersion into the zinc phos, but daub is the next best thing.