Originally Posted by MartyZ
I hate to dissagree with you but the original shifter and derailleur are indexed. I only replaced the original derailleur because the spring bracket is broken.
OK, I guess there were 6-speed indexed shifters back then.
I assume the original shifters and freewheel are both Shimano. If either are a different maker, that would explain why the new derailleur won't work properly.
You are going to have a tough time being fussy about the freewheel model you replace it with as the selection is now rather limited.