Just a comment on the practicalities: getting a definitive answer might be difficult. I think I've read the entirety of WA firearm related laws and haven'tThe current WA AG has stated his office will not entertain questions clarifying 594. And the AG generally won't answer questions from private citizens; you'd have to get one of your state reps to ask for you. The point here being, I think any definitive answer is likely to cost more in your time than the transfer fee.
As far as federal law, the closest think I'm aware of are the DEWAT rules for NFA items (because, from a Federal perspective, what's the use of making a normal firearm a non-firearm?). Here are a couple of references:
https://www.atf.gov/file/58166/download
"Deactivated War Trophy (DEWAT) firearms are still firearms under the NFA, but have been rendered unserviceable (i.e., incapable of discharging a shot by means of an explosive and incapable of being readily restored to a firing condition). The deactivation may have been accomplished by various means such as (but not limited to) welding of the chamber, cutting the barrel/chamber/breech, plugging the barrel, welding the bolt to the chamber, or some combination of these actions which rendered the firearm incapable of firing a shot.
Again, this is NFA specific, but if I was contemplating what you desire that "are still firearms" part would give me pause.
https://nationalgunrights.org/resources/nfa-faq#DEW
"By definition a DEWAT is an unserviceable gun that has an intact receiver ... could be removed from the coverage of the NFA if it was rendered unserviceable by steel welding the breech closed, and steel welding the barrel to the frame. All this was to be done under the supervision of an ATT inspector."
"under the supervision" is another troubling phrase.
I would be nervous doing what you propose. Sure, a judge might tell the prosecutor 'feh, it was all welded up, that's not a functional gun, get out of my court', but the judge might also say 'well, I feel a bit silly doing this, but it's what the law says, guilty as charged'. And finding out which will happen seems likely to be a lot more trouble than the background check.
You might try a letter to the ATF, with a clear description of the welding you plan to do, asking if that will render it not-a-firearm. If they say definitively yes or no you're better off than you are now.