Picked up a 75 TA, 455 HO, 4-spd about a month ago.
Working thru a bunch of minor issues.
Speedo was replaced with a 160mph unit.
Local street here has a radar speed check
Monitor and when I checked the Speedo it
was reading approx 10 mph slower.
Any ideas?

10mph at what speed and is it consistent? Doesn't matter, really. You'll need to know at least some of the variable and ratios before knowing which are incorrect. Confirm the rear axle ratio, can turn the wheels and count or if a new purchase flip the diff cover off and give it a fluid change, then record the details. Likewise with the transmission, unscrew the speedo and see what colour gears you have and go from there. Also, make note of current tyre diameter compared to stock and what is used in any tables you go off.

thanks guys-
Most likely issue is with the rear end, it is not a 3.23, got in touch
with a previous owner and he thinks they changed it higher... Going to take to a speedo place in
town and have them check it out, prob will need to change the speedo gear. This prob is down on my list.
Can use Waze when in car to check my speed. thanks again for help.

Given that you don't know existing speedo gear/s or rear gear or tyre diameter etc, there's many chances that you won't get away with just one change to make it accurate. All you can do is check and make a note.