Sorry guys I have been in hospital for a little time.
Here are a few pictures of something like what has been suggested except I tapped the original blocks after the guides are press in.
The picture represents an M6 thread which should then allow any sort of addapter to be made and then screwed in to take either the exhaust or a carby mount of the groups choosing.
This way there are no real changes to the original parts and if an exhaust or carby design needs to be changed you only need to make the appropriate addapter and screw it in.
As for those two small bolts that Dave is concerned about, I think that due to most of the parts being ally these bolts wil be more than fine.
Still just need a design that everyone wants for an exhaust pipe and carby.
If you would like to see some pictures just send me some info.
Then when everyone is happy I can whip up some workshop drawings for everyone.
Hope this helps.