Shane,
Not an easy job at all, however you could always track down some earlier doors - my 1975 coupe does not have the side intrusion bars. If you really want to save weight and have some (well a lot of) spare cash, there is a chap in Belgium Guy Morenhout who makes GRP/polyester doors/bonnet/boot lid and wheel arch extensions for Beta coupe. I keep looking but the cash flow isn't quite up to this yet.
http://www.abarth-gmr.be/new/parts.php?dzial=LANCIA BETA COUPE / MONTE CARLO#p_192
Slightly off topic I know, but I seem to remember you acquired a rollcage, where did you get it from?
Regards
Ian