Did some research and found GM used 0-30 ohm fuel guages in 1960-1966. The aftermarket tanks use 0-90 ohm senders and 0-30 aren't available. After some expermenting am installing a 41 ohm shunt to ground in parallel with the sending unit. Works very well.

The Standard TS-6 sender drives the factory gauge to "hot" reading when actual temperature is between 160-165. Some expermenting has given actual values for the gauge at 'cold' 'midway' and 'hot' positions. Have checked two senders and so far a VDO sender for a VDO gauge seems to match the curve best. Got the GM part mumber for the OEM sender and will try one of them if available.

You're right about GM not using regulators. Took a while to find that out. Thanks for the info.
Jim