Re: evinrude 1982 70hp
If I understand your cooling circuit correctly, water enters the block from the exhaust manifold, passes through the block and into the top-of the inner cylinder head (where your tell-tale now is). water exits the inner cyl head through the bottom and sits in the outer cyl head until the t-stat opens.
It sounds like you have a blockage in the cylinder head.
Did you install the tell tale with the head removed? It makes a crap-load of shavings....maybe they got washed down and formed a plug?
Was your impeller & intake screen intact when you 1st serviced it?