My advice (for what it is worth, maybe not much) is to go for the smaller option which will get you on the water quicker - and especially because it is a centreboard boat in which centreboard adjustment, and position of crew weight are significant factors in getting the helm balance right. In other words, "CLR" and "CoE" (so-called) are a lot less critical. If you position the new rig so that its geometric centre is not too far away from the geometric centre of the old rig, you won't go too far wrong - and just shifting your weight a little, or adjusting the centreboard a little, will be enough to get the helm balance how you want it. With a small boat you can't really go too far wrong, you will learn a lot and you will be confident when the time comes to tackle a larger conversion.
Summary: In the case of a dinghy or small centreboard boat, it is normal for weather helm to develop as the boat heels, and if it becomes too much you can reef (that's the beauty of junk rig) but in any any case, you can raise the centreboard a little, or shift your weight aft a little - in order to take a bit of weight off the helm - and that is normal.
If you choose the option of converting a larger boat with a fixed keel and more displacement - then it is certainly wise to be a little more careful about rig position etc (though I would maintain it is an art rather than an exact science) and it would be worth taking the advice of someone experienced, like Arne, before committing to the rig. There is more at stake, and perhaps it is not so easy to compensate later if the helm balance ends up not quite right. Even so, with some rigs it is possible to adjust the rig a little bit, later, to get things just right. You have done the right thing by providing a decent drawing of your proposed conversion - and by considering its position in relation to the original rig, which is shown - and by noting that the boat tends to have weather helm under its original rig. Hopefully this will attract some good advice from people who have had experience converting this type of boat. (To me, the junk rig looks to be a bit too far forward on your drawing, perhaps reducing the mast balance, or shifting the whole thing aft a little, might be better?)
For reasons given above, weather helm is not just a matter of rig position - the shape of the hull and rudder, and angle of heel all contribute. I would be wary of over-compensating by shifting the rig too far forward. It is normal for weather helm to develop if a boat is pressed too hard - and too much lee helm is much worse a problem than too much weather helm. If a helm balance problem remains AFTER the new rig has been fitted, you can still consider things like a larger rudder, or an adjustment to the rig itself.
As a learning experience and a prototype, I would be more in favour of the smaller option. There is less at stake, you can't go too far wrong with rig position - and structural considerations will be far less of an issue also. However, looking on the internet I could not find a decent drawing and original sail plan of the Potter 19.