It would/Should not be allowed unless the 80's swing arm is identical (same part number) to the one your replacing which obviously it isn't. Sorry but i would be disapointed if it was allowed especially in pre 78. You might be able to twist the rules to sneak it into evo though.
The other option is to build your own or get GMC to do one. That way you can almost built it how ever you like but just make sure it looks fairly period and similar to what aftermarket swingarms were around then.
You need to ask your self, were people racing in 77/78 with a swing arm off a 1980's model? No they were not. Thats an easy way to work out if a part is legal.