This has been discussed in the past. Whether or not you could use the gear legs that Gene Plazak designed (if you can find them) or build your own design would be a question. I think you should be able to mount them to the AA-5, but other questions would come up, like would this put the main gear in an optimum location for the AA-5 airframe? I think they are too far forward on the AA-1, and talking to Gene 15 years ago he agreed saying the FAA made him do it to prevent nose-overs. Then the question of the AA-5 fuselage being longer, what kind of loads does mounting a tailwheel on it bring? Seems like the AA-1 airframes with this mod have held up well (there is one based at my own airport that is an active flyer). Being the AA-1 and AA-5 are pretty much the same construction and the same materials, can the AA-5 take the beating back there without additional structure? And the heavier weight of the AA-5 needs to be considered.
Dave Fletcher said the STC was sold to someone on the East Coast after Gene past away and I have seen a couple of adds for it a year or two ago. Dave should know who to contact for it. That was STC SA7952SW.
If you try to go that route, keep us update on how that goes!