Wow, that's a cool project.
Having done the PRR D16 4-4-0 as a similar 'degree of difficulty' here's some things to consider....
1) The prototype had the advantage of using the trailing and lead trucks to stabilize. In N you want all the weight on the drivers only, and on that one, it's REALLY going to want to 'teeter' on the drivers. So the drivers have to be perfectly cocentric, square, and tight or it's going to bounce and hunt all over the place. I had a real problem with that, as I was swapping drivers on axles and only "Narrowminded" Mark was a good enough machinist to redo the drivers for me so they were perfectly mounted. Balance on the drivers is going to be hard.
2) With only 4 drivers, you can't really have a traction tire without getting something out of round and introducing a wobble....
3) You really want to consider, at least investigate, moving the motor to the tender so that you can pile all the weight humanly possible in the boiler. With an RP printed shell (I used brass on mine) any chance to use the body shell for weight is gone. And you can balance the weight easier. See my old D16 thread on how I moved the motor to the tender using NWSL universals and shafts. And if the motor is in the tender, you can consider a gearhead reduction; ideally about 5:1 if you can get close.
4) The tender will be critical for pickup. ATSF models has some good prototype tenders - see what Dirk has as he's home-brewed all wheel pickups. But somehow you need to get 8x8 or 12x12 in there. One of his tenders solved the pickup issues on my Jamco 4-6-2. And if you have all wheel pickup, lead in the boiler, and the motor for weight in the tender, the entire drive design starts to work much better. My D16 runs exceptionally well. I've done three tender drive conversions and I'd do more.
5) Watch the position of your drawbar to the tender. With that short wheelbase, if you put the drawbar under the cab, it will 'crab' the drivers into a curve against the outside curve rail increasing derailment potential as it will always want to climb that outside rail. See if you can attach the tender drawbar like Kato does; either off the trailing truck under the firebox or on it's own connection to the trailing truck screw. I had to do that to my LL 2-8-4, crabbed terribly and consistently derailed on curves until I moved the drawbar as close to the rear driver as I could.
I'll be watching this one. That RP printed boiler is exceptionally well done.
You're wise to accept the driver spacing 'as is' no matter what. The drivers have to be perfectly quartered, and the rods the exact length of the axle spacing, or you're in for a major battle there as well as it will 'thrash' the drivers slightly in revolution, again introducing a wobble. I started with the driver spacing on a Bachmann 4-4-0, so I could use the rods even when I changed the drivers larger.
It took me well over a year with testing the mech until I beat all the gremlins out of it, including an outright surrender to Mark that I had to admit I simply didn't have the skills to get those drivers right. By comparison, scratching the body out of brass and parts was easy.
