Glad to help out. The stock viewer is nice in that it folds up completely, but I find it hard to use without a +2 close-up lens as a magnifier. I built a small panel with one of those 52mm +2 closeup filter in it, to hold the viewer open at the top and also provide magnification. It's a bit clumsy but works well, though has to be removed and stored separately (would be nice if it folded into the stock viewer but I haven't quite figured that out yet.)
The prism is a better solution in all ways of course, but heavier and without 100% coverage. the RZ67 prism is the biggest I could find coverage-wise. (the Fuji GX680 doesn't have a prism viewer).
The 3x4 to 4x5 conversion is not too bad, though film coverage is only about 4x4.25 or 4.5 at best. It can take a lens of about 180-190mm as a minimum without modifying the mirror. I use it with a Pentac most often, or a Nikkor-T 270mm. (the 3x4-4x5 converted one with the prism). Theoretically it should be possible to add power (4LR44) and a tiny bit of other electronics to the RZ67 prism and have the TTL metering actually work, but I haven't attempted that yet.
Good luck with a DIY conversion - it's not that hard, if you are handy.