Thanks for the prompt reply.
I thought I might overcome the drum skins issue by sandwiching a circular baking/pizza tray, drilled to allow the wheel bolts and hub to pass through, between the back of the wheel and the adapters. Sounds a bit Heath Robinson I know but I think it could work!!
I'm still not convinced how robust bolting wheels to adapters which are in turn bolted to rotors, would be. Bolts would be pretty long too.
The alternative is to source some 356c/early 911 steel wheels or switch to 4 lug rotors and fit a set of JustKampers Beetle smoothies for a really early look. Both styles would bolt straight on. With hubcaps fitted the difference is minimal but I'd hoped to run without.
Here's a picture of my car. Some way off being ready for the road but the engine is built ready to fit, suspension, steering, gearbox and brakes are fitted, so it rolls and I have 95% of everything I need to finish it. What I seriously lack is time!