Thanks again guys:
Tunaman - the diagram kept me on the right track reassembling the pump.
Spare - I'm glad you reminded me about turning the prop counterclockwise to line up the spline and hold it in forward gear - went together very smoothly.
Got it back together and ran it on the earmuffs for a half an hour and it purred like a kitten and held about 140 degrees on the temp gauge. This is the one that was running for 20 minutes and losing RPM until it died.
I can't wait to get it on the water for a real shake-down but it looks like it's fixed.
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Scott, Portland, OR '85 V-20 I/O, Merc 170