29 years after a rebuild....

Spring height w/o retainer or shim is correct.

Can't see any re-conditioned head flowing any better; they don't port them. (Maybe you are thinking of something like EQ heads? n That won't happen without a number of changes....)

Seats may be needed on both sides. If the old seats were just ground down, then they all may be down in the heads too far. Re-con heads are a crapshoot for what you get there, so I'd just as soon take my own to a good shop and have them reworked and the valve heights set properly. (You might even have the throats below the seats turned with a Serdi cutter to get things smoother for a bit more flow.) May be more $$ that way, but you know what you are getting in the end for sure.

And of course there may be some wear in the rockers/shafts that is part of this situation.