It would appear that the U8s were AS.4s with the new engine, tall fin & bubble over the rear cockpit.
The U-9 Drones were standard ex-AS.5s - only external mods were the small wingtip pods.
If putting that bearded engine on a Spitty you would need to enlarge the fin too .... but what Mark of Spit would you start with ??, surely not a Mk.Vb.