temp gauge reading different now

[72jimmy4x4]

I put in a 350 motor out of later 70's in my 72 jimmy. The 72 heads had what looked like 2 temp senders 1 on each head. The later heads only had one and a plug in the other. I tried to take the plug out but ended up breaking my 3/8 breaker bar so I left it. Now engine in truck and running good but when engine hot temp guage only reads just in to the normal range. It was a 180 thermostat which is new. Thought it my have the thermostat so it got another new one.Finally there is it because there is no second sending unit?
Thanks for any help