The actual sidecar isn't a great issue, although lighter-weight rigs would suit a R60/5 while the R75/5 will better handle a heavier chair. Its about the horses and the limitations of the original solo; factory 46 vs factory 57. Don't be forgetting that a 36 y. o. bike will have less hp than at its debut.
Lower the final ratio (fit R60 to R75, & R50 to R60). On a chaindrive you just fit more teeth on wheel sprocket. Rather than changing the Hypoid ratio one can change the rolling tyre circumferance to a smaller size.
Reinforce the frame. /7s (particulary last models) are more crossbraced than /6 and /5 models. Some fit a subframe.
Reduce the trail at the front wheel. If the original geometry is retained, steering effort will be high as you are fighting the drag of the chair reinforced by the self-centering effect of the trail measurment. This factor affects trikes & outfits because they do not (or should not) lean in corners. That's excepting outfits like Equaleans, Flxibles & Armec Sidewinders.
Changing to Leading Links is one method (and quite sturdy if well built), but not the only way.
When an outfit is built, and has the geometry set suitably, there still remains re-training the driver! Many an accomplished solo rider has come to grief trying to pilot an outfit!
Why not peruse threads here at ADV to learn what chairs are fitted to Airheads? There's no RIGHT or WRONG choice really; its more whether the execution of its design & building is adequate.
'77 BMW R100RS with Ural chair
'08 Suzuki AN650A Burgman (and trailer)