Water temp and gas gauge not accurate while engine running.

I got my my engine back in my car and running. Before the install I had a problem with the gas gauge being accurate with the key on, but as soon as I started the engine, the gauge would read a lot higher. Now both the gas gauge and the temp gauge do it. The temp gauge makes me nervous since while driving I wont know how hot it is.

A little background.
The original dash has aftermarket gauges in it. It had the original firewall forward harness in it, now it has a slightly used year one harness, and MSD Digital 6 Plus ignition. The year one harness is not for a 73, it's a big block harness not sure of the year. Everything seems to work fine except for those two problems.

Any insight would be greatly appreciated.

Thanks,
Slim.