Nah, you really don't need 12000. A 10K would have been a better choice but I don't carry that one. I really should look into that size. Something like my CER-10000uF-25V would be decent capacitance-wise. But, being a standard radial cap - these don't work nicely with the cap clamps nor the existing wire attaching method.
The 6800 is actually running a bit small but the System 80's really didn't draw much power anyways so ... regardless of how small the value seems, they do work.
You could get by with 6800's but I would avoid the cheap Chinese caps in the ebay link (there's a reason they are that cheap).
This one is a nice, long life cap ... but is already ~10 years old:
ebay.com link » Cap Alum 10000uf 25v Radial Can Solder Lug 28 Mohm 100hz 15000 Hrs 85 C
Fortunately, 10 years of shelf life is much easier on a cap than 10 years of 'in use' life.