Welcome to the new and improved Proflex / K2 Tech Forum!
you are right the originals had two different lengths.I would try the new ones as they are.There are some alternatives to K2 elastomers now,there is a guy SLIM who has made some,and I think Callan aka RDI has some too.
You mean Callum at Rapiddescentscotland (www.rapiddescentscotland.co.uk) ;-)