The first step should be to match the transistors in the differential input stage for equal hFE
For the CA-410:
They are TR-405, TR407 in one channel, and TR406, TR408 in the other channel.
Then you must trim R443 and R444. If the initial offset is positive, you must reduce those resistors by small increments. If the initial offset is negative, replace them by larger value.
A 2k trim pot should work fine too.
For the CA-810:
Match TR403, TR405 together, and TR404, TR406 for the other channel.
Trim R423 and R424. You may replace them by a 150 Ohm trim pot. It should provide enough adjustment.