Sunti, perspex is fine or polycarbonate, you only need the film on the side windows if they are indeed glass. You need to check the Homolgation of the Sunbeam as it was in 79, as this is Classic, you still actually have to run historic wheels and tyres. But there is talk of the tyres freeing up a little for next year. If the car is running as a TI it will have to run to what was homolgated in 79 and as far as I know that was Drum brakes on the back and std box. But again, try and find someone with the Homolgation papers for the sunbeam and there you will have a clearer picture.
Appendix K of the MSA rule book is what we use in Ireland as our rules. Again though there has been talk of maybe the tyres freeing up a little as we can only run road legal tyres and 70 profile no lower.