Since no-one else has yet replied, I offer the following comments and ask some questions, in the hopes that further discussion will follow.
If all the engineering is sound, this hull, with its rather shallow draft and tandem centreboards looks like an exciting and attractive proposition, especially for junk rig. A lot of shallow draft hulls tend to develop weather helm when sailing off the wind or when hard pressed, and junk rig can exacerbate that - but with that aft centreboard down it should track down-wind like a train on rails.
Normally, where there is an existing sail plan from the original design, which has been proven satisfactory from a “helm balance” point of view, it should be straightforward to build a junk rig around the centre of area of the original sail plan. In the case of a split junk rig, the new centre of area of the sail plan can coincide with or be very close to the original. In the case of a contiguous (unsplit) ie conventional junk rig, there may be a slight adjustment on that, as you will see from Arne’s notes, but the point being: a successful original sail plan gives a confidence-building start-point for the design of a suitable junk rig. Evidently you have followed this principle, and placed a junk rig which has taken into account the drawing of the bermudan rig which the previous owner has given you.
To my eye (and I don’t claim to be an expert) – the result does not look correct.
The centre of immersed lateral area of the hull (with both boards down) looks to be about where it is shown on the drawing – well aft of what is normal (but with the ability to adjust centreboards this can have a positive advantage especially if this is the type of hull which easily develops a lot of weather helm). However, the centre of area of the sail plan seems set so far forward in relation to it, that I would be concerned about possible lee helm, or even difficulty tacking – I imagine sailing to windward with the aft board fully up and the forward board fully down and still wonder if the rig is a little too far forward. The large mizzen you have drawn might compensate for that, but it still does not look right to me. (On a full ketch the mizzen is meant to be a working sail, not an air rudder).
The other thing which would worry me a bit about your proposed junk rig is having the mainmast itself placed so far forward – ahead of and over-hanging the waterline in fact, with barely enough bury for the integrity of the mast, in addition raising the question of whether the hull itself is structurally capable of supporting a free-standing mainmast at that point – together with the effect this new weight distribution may have on the vessel’s tendency to pitch in a head sea.
In summary, from an unqualified lay-person’s perspective, the combined centre of area of the proposed junk ketch rig "just looks to be" too far forward, and in addition that mainmast also looks to be placed too far forward.
So, I ask the following questions: (1) What is the intended purpose of the vessel? (Live aboard? Coastal sailing? Canal cruising? Long distance ocean cruising? For how many people?)
Not wanting to be negative. This shallow draft, capable-looking long keel vessel with its tandem centreboards strikes me as a very exciting proposition with a lot of possibilities. In my younger days I would have dreamed of owning a vessel such as this. If the hull engineering is sound, I’d say go for it. Perhaps some discussion with the original designer regarding the original rig would be helpful, especially regarding its handling characteristics - and get some good advice from someone qualified before settling on your design for the new junk rig.
PS on second looks, that bermudan sail plan as a sloop, with just the 3/4 headsail, looks as if it would go quite nicely hard on the wind with the aft board up and the fore board down. Perhaps that was meant to be the working configuration in heavy weather. Down wind, you could cram on as big a genoa as she would carry, and with the aft board down (and maybe some of the fore board up) I can imagine a good down-hill ride. Maybe this was the rationale for that rig.
I just found on the internet the sail plan for the original Trismus 37 with a bit more detail. It seems to be a cutter rig and and it looks to be the same sail plan as you were given, but with a bit more detail.
Anyway, for a junk rig, I still think the mainmast wants shifting aft from where you have placed it, and perhaps a junk sloop (or a two-masted junk rig) with centre of sail area moved aft (from where you have it) might be better. With tandem centreboards to play with, you have a certain amount of choice. You really need to find out more about the boat's characteristics from someone who sails one of these vessels. There seems to have been a few different hull and rudder configurations.
To my amateur eye, its an unusual and interesting proposition.