OK- the late Super Record rear mechs, shifted to the 28 better.
Check Velobase.com for visual reference.
That out of the way, back out the adjusters, you probably have the short dropouts as its a 1980, I would consider removing the adjuster screws and springs temporarily and slide the wheel aft, essentially all the way.
It will help, but if not enough... then alternatives are:

Shorten the chain the minimum amount ( with a gloved hand) twist a link to simulate a shorter chain along the return run of chain between the chainring and the jockey cage.
Watch the cage rotate away from the cog. The test prior is to go to the big/big combo, interestingly I have a bike that to get into the 28 (as I do not intend to keep that and plan going to a bigger big ring)... Must be in the big ring to accept the 28T.
I do not advise setting up the bike so it gets in trouble in the big/big even though you would not use that, duplication and friction.

So, try that and report back.
If not we will go to phase II.