sounds like the way to go, order it then walk into the guys who wouldnt do it for you with the longer chain.Send the 130 chain back and get a 140 link chain from DID with a rivet link(they do make it, a 520VM). The problem is not a lot of people make 120 link + chains in 520.
and i have never run an extension, but i normally run 2 masters on track bikes so i can play with length easy. never had an issue. i have however had a shop put a clip type master on backwards, hit 7k rpm and it broke my case.