I've been working with Tomytec parts now since about 2006, first chassis I used was the TM01. The truck design comes in a couple basic flavors- the original truck (that I use) that has the gear towers over the end, and the "R" revised version that has the gear tower in the center. For this project, it doesn't matter that much. The second major split is on the truck wheelbase, they have a long wheelbase and a short wheelbase truck; long wheelbase truck has end-axle pickups, short one has interior pickups on a plate that rides on the axle. All of them have Atlas/Kato-style sprung pickup wipers. Quality of the materials is comparable to Kato, generally good stuff.
Above the trucks, it's a free-for-all on chassis length, it's a plastic frame with a motor with a plastic driveshaft. Almost always 1 or two flywheels, pretty much now a standard motor. So while there are umpteen different TM frames over the years, the trucks / motors are relatively consistent. I've been able to fit all manner of trucks and motors into the frames I designed, so that's what I recommend - don't design to a frame length, design your own frame that just holds standard trucks, and I splice driveshafts to length with K&S tubing to fit your exact situation. Problem solved on all the odd chassis lengths. And the entire chassis is a snap-together design, you can field strip one is about 3 minutes flat to parts.
And, while Tomytec has 'pizza cutter' flanges, it's a split axle design, I chuck them in a dremel and reduce the flanges with a file, I've got it down to where I can do about a wheel a minute, checking with a micrometer. Easy as it gets.
The Plaza Japan online store has a big assortment of chassis at a far better price than Ebay, and they make a practice of including a scan of the chassis dimensions and good photos so you know what you're getting. It's a pretty evolved solution, really. These and the Kato 11-105/6/7's are the backbone of what I build off of.
