Temp problem.

[HTX86chevy]

I did a 305 to 350 swap a few months back. I threw brand new parts all the way around to avoid having anything fail prematurely. I put a brand new radiator, matched it to my old one. Threw in a brand new temp sensor, the one on my 305 was too small to be reused. I got a new 195degree thermostat and radiator hoses.
My temp gauge is going all the way up to 260 degrees while idling or drivingo for 10 mins. its about 50* outside today and I cranked her up and let her sit for 15 mins and she reached 260 on temp gauge. This is also the second temp gauge I have installed thinking that the 1st gauge was the problem.
any ideas?