From 1987 to 1989, there was an aluminum head 2.8 engine that used valves very similar to iron head 2.8 engines. They are technically shorter, .020" but that won't make much of a difference to require new springs and pushrods... The big advantage is that they have 8mm diameter valve stems. Stock Fiero 2.8 valve stem diameter is 11/32". I am building a set of heads now for when my 2.8 finally dies and my machinist was able to find these with the following part #'s: 201057 is the intake valve and 201058 is the exhaust valve. I don't know what supplier he has though... For guides, he was able to look up the iron head 2.8 specs and get the correct guides. He said at worst, he's have to shorten the guides to fit.
Next time you are rebuilding or refreshing heads, I recommend this route with valves. As an added benefit, they are super cheap. ~$8 a valve.
That's a good bit of usefull info. You might ask your machinist to back cut the valves while he is at it, if he hasn't already said he was going to do that. It enhances low lift flow and would compliment the narrow valve stems. It' not expensive to do.