I have a 98 353 fastech and 1 motor is telling me it is hot like 190 f at 4000 rpm. The gauge will climb and jump back to 160 or so instantly. Never any alarms going off. I shut it down and felt the hoses. The hot one felt warmer than the other, but not concerning hot. I'm not sure I believe the gauge with it jumping around. Sending unit? where are they?

Starboard side of the thermostat housing. Single Brown wire with black boot type connector.

Swap your temp guages

Good idea, how do I do that?

Physically move your port guage to the starboard guage and vice versa. If it moves with guage swap its your guage. If it doesn't it's either real or sending unit. You can also pick up a temp gun and shoot motors. Good luck.