I'm usng a cambo rear standard as a front standard right now. I use 8x10 to 4x5 converter boards (without the 4x5 standard--just the plates). These can be used as lensboards for LARGE lenses. Or, you can machine out the center and make a VERY stout bellows frame that can acutally be used to screw a bellows in, rather than glue it on, like stock cambo bellows. Oh. These are the PERFECTsize to JUST fit a packard 5" hole shutter on the inside too. the packard frame just fits insid the front standard of a cambo. So there's an alternative for you.
I have extras. I also have an extra 810 bellow set with frames, but these bellows are good users. Shame to wreck them, besides, I may need these--one never knows, right?
I'm using my large front standard regularly right now and I may need these things to make converter boards, packard mounts, lensboards for large lenses, etc. They are not that easy to find at a reasonable price anymore, it seems. I don't need money, so right now I'm not interested in selling, but maybe if you have something that I want, a trade can be arranged.
At any rate, it's another alternative for what you want to do, perhaps. Maybe you can do a wtb for a converter back like these and have more success getting resuilts. There are far more of these around than extra bellows/frames sets.