Excuse me if I call them RD's. I was told they were "changers."

OK. I bought a 1989 Chorus group from a guy who rode it fairly often. It included 7-sp DT shifters with the blue insert, a Chorus RD, and a Regina 7-sp freewheel.

I put the freewheel onto my Record hub laced to a Sun wheel, no problem.
I mounted the shifters to the downtube of the bike, no problem, got the right # of clicks, no problem.
I mounted the FD and RD and the bike looks great. I adjusted the RD and the chain likes to jump from the outer cog to the 3rd cog. I did some adjusting, but since it runs from #3-#7 no problem, I left well enough alone. It shifts fine in friction mode (pull out the knurled knob, twist, etc).

The bike is still skipping the 2nd cog from the outside. I took it to a bike shop, and the mechanic said that the insert is incorrect, it should be a B model. I've looked for one, pretty hard to find part. Another mechanic said if I swap out the RD to an Athena, Victory, or Croce, pre-Ergo, it will work.

Since the gap between the outer cog and the next one in seems to be the problem, can the outer cog simply be taken off and a spacer put in, or is it more of a problem with the RD and/or spacer? I've not had a freewheel apart, and wouldn't want to mess it up.

I'm much more used to cassettes, and is it possible I can put a freehub on it and build a cassette with a little more gap between those two cogs to make it work?

I know the Syncro stuff was balky, and I'm sure the bike's rider (my relative) could make it work, but I'd like to remedy the problem.

Second issue: I have an extra one of those inserts and an extra set of 1st generation Ergos. If I put that insert into those Ergos somehow, would that create a 7-sp Ergo, given the right RD...it looks like the part numbers for the 8-speed inserts are the same, and I was wondering about moving "down..."

Thanks.