a couple of years ago I was caught in a heavy rainstorm and my model 70 was drenched; when at home I removed the barrel and action to dry it out; removed firing pin etc. reoiled firing pin after three days and reassembled barrel action; today, the hinged floor plate will not open up; I suspect the wood swelled and I don't know if I'm capable to remove the swelled wood or if I should take it to a gunsmith; should the inside of the gunstock be coated with a stain or sealant to prevent this from happening again?

Yes.

#1 - Check the latch to see if it has rusted in. Oil as needed.

#2 - With the two-piece bottom metal, I have found it is necessary to adjust the trigger portion(with the floorplate latch) while screwing it on. You can move it a bit fore and aft to allow the latch to work properly. If that doesn't do it proceed with the 'smith.

Good advice there!

Yes but be carefull with that adjustment as too much play, and if its a big bore especially, you will have cartridges all over your feet from release due to recoil....Ive seen this happen more than dozen times, even one of the guns I built for a friend did this in a 404 Jefferys, had to replace the original spring with one a tad stronger..