Good to see you got it close to finished. When I first ran my Sunny coupe I had problems with it overheating & blocking the cores of the radiator. It wasn't until I fitted a filter inside the top radiator hose that I got this under control, as every time it overheated, it brought more rubbish out of the block. When this was fixed, I noticed the problem with the standard temp guage in the car. On advice I used a standard A12 temp sender in the engine, which fitted in perfectly. Try grabbing the one from the original motor, but they sometimes cold weld themselves in & removing them destroys them. The part number I have for the new sender is TTS1065, though it's likely they'll have to order one in for you. As for the overheating in traffic, I was advised last friday it's more important to get air flow to the radiator. So if you can trim out the radiator support panel to expose more of the radiator core, this will help.
With my ongoing L20B swap, I'm using a Stanza 2 core radiator that I've rebent the sides where it bolts to the support panel, so it sits hard up against the panel, rather than have a big gap. Hopefully I'll be able to fit a slim 12 inch thermofan between the radiator & the water pump, which I'll control with an N12 pulsar switch installed in either the lower tank or lower radiator hose.