It appears your bike would use the "adaptor claw" type of rear derailleur one that aligns with the drop out rather than screwing into a dedicated derailleur hanger, pretty standard on entry and mid level 10 speeds of that era, I would suggest find a local bike co-op that sells used parts or what I do with some of my builds is use the SunRace RD M10, not too expensive on Amazon, looks the vintage part and its brand new, I reuse the vintage/original parts when ever I can but sometimes you just need something new to get it all working. Someone may have already mentioned this but if you go to Sheldon Brown site he has a nice article on chain sizing that I have found helpful and lots of other great info too. Good luck with the build and have fun! Love seeing old steel get new life.