I`d guess backing plates should be fine for a template. The length of the gearbox input shaft should be what determinds the thickness of the plate. Remember threads will be cut in the outer holes. Thread engagement probably should be 1.5 times bolt diameter. The bellhousing bolts are 8mm, Threads for the starter bolts are 10mm or 12mm. I would think 15mm should give enough bolt engagement.

If they are going to be milled out, ally will be cheaper to make.

I wouldn`t be ordering anything until you have at least made sure you have all things covered in full.
Things like, Hydraulic clutch activation(master and slave), where and how the input shaft will spigot, Clutch and pressure plates and if it will fit the tunnel without major surgery.
Tailshafts and gearbox crossmember have to get made aswell, but they can be sorted later.

With the spigot bush location, it can be also be put in the flywheel insead of the crank. So if the adator plate makes the input shaft not reach the crank, its not the end of the world. This may also mean that the 71b input shaft may not have to be machined to fit the original spigot bush.
So either the shaft will have to be modified or a new bush made or maybe both.
The other thing is ensuring the shaft spline is in the right place for the clutch plate once its all bolted together.

The adaptor plate is probably the easy bit.