@YorkshireDave might be able to advise on the best type to use. It's a topic I'd expect him to be all over due to his interest in replacement tap cartridges.
Did someone call ? (imagine the Bat hot-line ringing....) Hmm. Can anyone spell shambles?
First thing that occurs to me is to ask how long you intend this to last Paul?
I would strongly advise you to get spares NOW. If my recollection is correct, they were not any form of standard such as BS5412 or BS 1010 fit.
Post 8 seriously concerns me. Stressing components like that will means something will give and it also looks like it is impossible to seal the tap against the bath top.
If having parts made, I'd suggest low lead brass - 2% or less. However, no matter what you have made, if you are stressing components then you can be 100% certain of some leaking and subsequent low term deep seated damage to your home as it those are by their very nature slow and insidious. You can expect considerable expense to rectify once discovered.
If you cannot fit the taps simply, and without interfering with other components, when the bath is in situ then you should abandon the use of one or other of the components that are conflicting.
I fully understand it's not what you want to hear, but it is the most sensible advice I can offer insofar as you will then have a usable, leak free bath where the taps are then replaceable once they fail.
Sorry.