Getting speedo to work

[70chevlow]

I recently got a 1970 chevy c20 custom camper that i did a engine swap on. Ive been going along trying to fix things that the previous owner hacked toghther. Currently im trying to get thw speedo to work at low speed it doesnt want to work and then around 30-40 mph it started moving eventually pegging at 70mph than started making a grinding noise. Any idea what i should check? The speedo cable functions hooked it up to a drill and it is spinning. Also this truck has a 700r4 trans.

[66pnl]

Just went through this week end. Had the same problems. Replaced the speedo with a used one for 20.00. Works perfect. Completely disassembled, cleaned and detailed. Fun project.