True submersing carb cleaner should do a great job if you can find it. Not the spray can stuff.
Alternatively you might have a radiator shop around that would clean it.
Have you tried shaking with BB's inside?
I have had good luck sealing tanks using RedKote. (also known as Northern. Same exact formula, different color)
_________________
Chuck.
“Any suspension will work if you don’t let it.” - Colin Chapman
Visit my ongoing MGB Rustoration log:
over HEREOr my Wankel powered Locost log :
over HEREAnd don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766