solved the issue of sealing the tank by just purchasing an inner tube for a wheelbarrow. it was the cheapest bit of rubber i could find anywhere.
So, i've completed the process of filling the tank with acid, soaking it, rinsing it out completely with baking soda and distilled water, drying out the tank with a heat gun, and trying to coat the inside of the tank with POR15. results are iffy. I certainly got a whole lot of the rust out, and it was very clean inside at first. After that it was a race against time to prevent flash rusting. i used a whole pint of por15, sloshed it around inside real good, or as good as I'm able to (you can't see anything inside obviously). it's now in the drying/curing stages. I've read it's best to leave it for 4 days minimum without using or filling the tank with gas.
trying to look inside the tank, it's not altogether perfect from what I can see. it seems like the tank sealer coating was not completely perfect, nor was the coverage of the acid. i filled that tank up right up to the rim, but it still looks a bit like the acid didn't completely scour the top of the gas tank. the surface on the inside up there is kinda rough, and it looks like there are streaks of semi-reddish material running down the now-dry por15 surface on the inside of the tank. I'm thinking this looks really iffy and maybe I've failed to do this correctly. I'm going to let it cure, then attempt to fill it with gas with two new fuel filters installed on the petcocks. If this does not work out properly, i'm not going to attempt it again. too much time and money was invested the first time around. for only a little bit more than the cost of attempting it again, i can buy a new gas tank with no rust on the inside.