I have a working A4 from 1967 in our Tartan 27.
I was told by the PO not to let the tank get too low (it is not like a car) as there can be crap or crud (technical name) at the bottom of the tank that will do worse things to your carburator or engine.
That said, it sounds as if your fuel tank could stand to be drained, cleaned out and aired out. They sell a liquid pump that attaches to a drill that you could use to extract the bottom of the tank. Repeated soakings with a little clean gas and/or Marvel Mystery Oil may help to get the crud out from the bottom.
I have never used HEET in our tank but the A4 is basically a tractor engine and could probably handle a bit of this stuff dissolved in gas. We routinely put some MMO in our gas and oil supplies and the engine does not seem to mind this, in fact it is a recommended practice by A4 guru Don Moyer. Speaking of Moyer, have you found his website yet? It is invaluable resource for all A4 owners. Moyer Marine Atomic 4 Community - Powered by vBulletin