Temp gauge question

[75sierraclassic]

I've got a 75 Sierra Classic that i rebuilt the 350 engine. By suggestion from the machine shop I put a set of 305 heads on the engine to raise compression since one of my heads was cracked. The 305 heads use a different temp sender than the 350 heads, so I bought a temp sender for a later model truck. Do you guys know if the guage in my truck and the newer sender are compatable? The guage only registers a little bit over the cold mark. It seems to be registering a temperature, but it is like the scale is off. I used a temp gun to measure the outlet temp on the intake manifold and it was your normal 220 F. With it registering so cold I really don't know where the engine may be overheating.