1991 Suburban 454 heat issues

[extd56]

I installed a 1991 Suburban motor in my 69 crew cab truck and put in Autometer gauges. I am having a problem with the temperature gauge reading hot. I had the sending unit in the right head and the gauge was showing that it is running 210 in the driveway but when I took it out on the freeway it went up to 250. I moved the sending unit to the thermostat housing and now it reads 180.00 all the time. Can the sending unit next to the exhaust make a differance in the reading? I am running a Edelbrock reverse flow water pump and a be cool radiator and it seems like it is running cool but I am worried about the reading from the head.