This is a "me-too" post, sometimes helpful in consensus building. First water pump I replaced I did exactly what you did, just before a thousand mile trip. I only had one brick then, so I had to turn around after 20 miles and find another seal locally quick, which meant buying a complete aftermarket water pump at the local auto parts store.

Installed o-ring on pipe with a bit of red silicone sealant mostly for lubricant. Smeared a thin layer in the water pump casting hole too. Carefully eased the pipe with seal into the pump casting. Trip was thereafter successful.
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Art Benstein near Baltimore

Put the new o-ring on the tube and then insert the tube into the water pump.

There is a clamp on the tube near the back of the engine block. Remove the bolt there, and you should be able to slide the tub back far enough to install a new o-ring.
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john

BTDT My procedure now is to put a little silicone caulk on the pipe and slide the ring into place. Apply a little more silicone and wiggle the pipe and ring into the pump. The silicone acts first as a lubricant and then when it sets up it helps produce a good seal.

Randy