I think at 70k I'd probably go the HD reman route. I considered it recently when my cam tensioners failed at 37k. The dealer told me $3680 installed but the other folks are right (I checked) - if you go reman, YOU WILL GET THE SAME OLD P.O.S. TENSIONER system. The reman motor comes w/12 month 12k warranty.
Gears may be the most durable fix but I was told by more than one wrench that if you have excessive pinion runout and you may with 70k miles, they will be noisy. They said they even had to remove them for folks who couldn't stand it.
I called Andrews and spoke with them at length and their tech was very high on the roller chain conversion and hydraulic tensioners. Said the new shoes are a much better quality and that the chain can better compensate for pinion runout. That is the way I went and it was $1650 out the door with cams, cam plate, higher capacity oil pump, adjustable pushrods, new lifters, and the rest of the sprockets, gaskets, spacers with SYN3 oil and filter (incl 6 hours labor). The stock cams are SE203s and I went with 37Ns which gave me a little more punch. I saw someone mentioned the possibility of problems with these hydraulic tensioners at 35k but both sources I talked to said they have examples of motors with 50k or more and no appreciable wear. My motor is now solid and quiet. I suggest you call Andrews yourself if you don't want to go the whole reman route. They even referred me to a independent who has put together an entire installation kit for under a $1000 if you handy enough to do it yourself. His name is John Wadworth and he can be reached at herko@insightbb.com you can tell him Randy with the SERG referred you.
Anxious to hear which route you choose and your results. I am always happy to hear of folks who go 60-70k plus miles with these older SERG's with the economy as it as, I suspect I might try to get mine (currently at 39k) into that same range too - one guy in our local club has an '00 SERG and just turned 100k!! He is a pretty competent wrench in his own right and says he has never gotten into the bottom of the motor. He had the same problem with chain tensioners at 42k and went gears (510Gs?) and did hp heads at the same time and has been happy with his decision these last 60k miles and his sees some rough duty pulling a sidecar 90% of the time.
Good luck,
Blue