The 23 spline is needed for the the amount of torque/hp that the 3.0 puts down as compared to the 22r(e)'s, which is why the 22rte(turbo) came with a 23 spline to start with.
The 3.4 uses the same transmissions as the 3.0, so the trans to gear drive t-case adapter plate should be the same, a call to Marlin would confirm if the gear driven to chain driven t-case adapter plate is also the same.
Running a single 4.7 case is an option, a lot of people do. I am not sure if it is a complete bolt-in procedure, the adapter plate may cause the need to relocate/modify the X-member and the drivelines. The cost of running duals on my '90 was one the main reasons why I decided to build my '91 w/22re.
Another option may be to run an R151f transmission from a turbo, then you can bolt up dual(23 spline) gear driven t-cases, the same way you would w/a 22re, and save the cost of the r150 trans to gear driven t-case adapter plate. This was the route the guy I bought my dual 21 spline t-cases from was trying to go w/his '89 3.0 equipped Runner. IIRC, he determined that he could use the r150 bell housing with the r151f transmission., a call/email to Marlin should let you know if this is possible.
Given the trail rating of Swamp lake and depending on your skill level, the single chain driven t-case along with proper gear ratio(5.29s for the 35's) should be low enough for you to get through with little issue.
