And "... digital projectors are only going to have extremely limited geometry adjustments, if any at all.. "
- I really had no idea about this. Since the G3 screen is curved, would two DLP projectors be able to project a seemless picture then? And what about the throw distance, are the CRT projectors generally better in this aspect? It would be a nightmare if you'd swap the CRT-prjs for some LCD/DLP ones, and discover that you cannot fit the size, being it too large, or too small.
Digital projectors are bouncing light off of or passing light through a rectangular panel -- all of them; DLP, LCD, LCOS, etc. The shape of the panel cannot be physically changed. What this means is that if your screen isn't flat, or you're projecting off-axis, you will not be able to get a nice picture with a perfectly even geometry. More expensive digital projectors will have the the imaging device mounted on motorized pivots so you can mechanically adjust the panel to be off-axis inside the projector's light path, so you can correct for off-axis projector mounts before it passes through the lens system. This still only provides limited adjustments; basically strictly "keystone" adjustments.
You could find digital projectors that would project a nice picture in a G3, but if you want something with a good black level (and you do), you'll spend 150x more than you would using old CRT projectors that will still align better.
Depending on the projector's electronics, a CRT projector can draw the beam wherever you want it to on the CRT face, so they're ideal for stuff like this. The Sony 10xx series are about the most basic projectors out there, and even those can be very easily tuned-up for good alignment.
Darth: you're going to have to intimately familiarize yourself with those projectors (or, CRT projectors in general) to really dial them in nice. There are probably videos on YT on how to converge the Sony 10xx projectors, maybe even adjusting G2 & color drives. Considering they're basically the most common projectors ever, there is a lot of info out there on them. You should do all of that..
http://www.curtpalme.com/Sony10xx.shtmAnd search the forums on that site as well; lots of good info on those projectors.
..but, if you have 2 spare projectors already, you should just pull the original ones and use those. The tubes are likely going to be in far better shape. I only suggest this because you really will have to just start with fresh projectors, one way or another -- don't keep those originals in there, they are too tired by now -- both the tubes and electronics will have 10-15k+ hours on them. ANY Sony 10xx projector will drop right in there. The model # isn't really going to matter; they mostly denote inputs/voltages, and are basically identical inside. As long as the input voltage is right, they will all display the video signal since it's just standard 15.7kHz.