Borrow from Peugeot of the same era - some were using the Shimano SIS stuff. Not prohibitively expensive to buy aftermarket. To pull from recent projects for example, the 87 or 88 Peugeot Nice is all Shimano A350. It'll give you the index-based stuff you want without the excessive cost.

If you want some extra $$ to purchase stuff by getting rid of the stock parts, keep me in mind. I go out of my way to restore bikes to stock, so I'm always looking out for stock stuff.