Temp gauge readings

I have a couple of questions. I have a 68 camaro with the temp sensor in the water neck . To get the electrical lead to the sensor I had to splice in a length of wire and soldered both ends. The temp gauge reads high--note that the sender is the right one for the gauge. I wonder two things:

1. Could soldering affect the line resistance and the gauge reading?
2. Is the water neck location significantly hotter than the original block location (which I can't use because the appropriate sender is 1/2 inch and in the newer motor the sender is tapped for a 3/8 inch sender).

Thanks for your thoughts.

Steve