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Nick,
A few comments:
The success of a mod, based on my ten years experience, is directly related to the quality of the etalon used.
The 10/80/10 rule seems to apply to all the etalons I've seen.
10% are basically rubbish and should never have left the factory.
80% are OK but....just acceptable performance - sometimes soft surface detail and loss of clarity on the proms.
10% are Whooooo -that's neat!!!!
If you use an average etalon in a mod you will not get the bang per buck - spending the $$$ to get the OTA/ ERF and larger blocking filter will just get you a brighter average etalon performance.
The Lunt mod involves removing the front objective and either using the remaining tube as an adaptor to the donor or making up a suitable adaptor to hold the etalon assembly/ focuser in the donor OTA.
Either way the etalon will act as an aperture stop and give an f7 outcome.
The scopes you mention, 120/600 and 80/400 would end up 85/600 and 57/400 systems.
Above an aperture of 60mm I strongly recommend either a full aperture ERF - the Baader D-ERF is excellent, or at least an internal ERF of >50% the aperture.
The blocking filter? I assume you have the basic B400? This will cause vignetting with focal lengths >400mm. For a "good" mod you'll need something like the B1200.
All this adds up $$$$$$
Doing a mod is not a trivial investment, and will probably not allow you to do full disk imaging. Think it through carefully.
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