John I couldn't recommend higher, the quality, price and workmanship from Colin Byrne. He made and fitted my cage to CAMS spec.
Trouble is, would he design and make one for a 75. You could ask him?

1)  Is a half cage an option or are they only acceptable for those that already have them?

Schedule J only allows a half cage in an open car (MX5, MG etc)  however for sprints this would be better than nothing even in a closed car.  You cannot do any event that requires a roll cage in a closed car with a half cage.  As i said better than nothing but does not comply with Schedule J.

2)  Is there an accepted appropriate design for our level of motorsport?

Schedule J of the CAMS manual, and within that there is a minimum design.  Again as there is no requirement for a cage at a sprint event and given the non-enforcement at sprint events of the requirement for schedule j cage where a cage is fitted, you could vary this design however I think to vary from the designs in the manual without an engineer is dangerous and would add/save nothing.  The minimum CAMS design is a good comprimise between weight and protection.  This design is a 6 point cage, bracing to the the main hoop, side intrusion bars and roof bracing.  I would also add bracing to the backstays and to the firewall.

This is the design I have used in a number of GTV's and GTV6's.  Don't forget that in racing events where cages are mandatory, racing increases the risk of rollover becuase of the increased likelihood due to cars being in close proximity - it doesn't change the consequence of the rollover - a rollover at a sprint event can be just as bad as a rollover at a race - it's just less likely.  A design that is minmum for racing is just as applicable for sprints.

3)  What are the pros and cons around weld in and bolt in cages?

Welded are stronger, bolt in can be removed but are much more expensive.  If the car is a race car, ie it will never ever go back to be a "road" car then weld the thing in.  Stonger and cheaper.

4)  Are cars road legal with cages in Victoria?

Yes, they can be but there are certain criteria - eg side intrusion bars must be no higher than the base of the seat.  The cage builder can advise - there is also a leaflet available from Vicroads - i have attached a version from 2000 which i think is still current (whoop's - file too big - can't attach - go to vicroads website).  Ditto Ed's comments.

5)  Would the recommended design change if during the remaining life of the car it may end up with a 164 or 24 valve V6 in it?

Probably not.  The basic design discussed above would be fine.  Schedule J applies across all classes of racing and types of vehicles.

6)  Who offers a good reputation and balance of product and price?

Agree with Phil.  Mark is a genius with metal and has done a lot of alfa's.

7)  What budget expectation should I be thinking of?

I'd budget $3k.

  What weight penalty do you have to accept?

For the basic design probably adding around 30-40kgs but you could offset that somewhat with removal of the intrusion bars in the doors.

Please offer your contributions to any of the questions you have an opinion on.  I tried a search on the forum without success, so don't think this has been covered recently.

Thanks,
John

Well I know he won't do a cage for a 90....