By modelling the trucks you’re adding a lot of calcs to the simulation but it isn’t the results of the trucks that you’re interested in. Best to model them as simple blocks imo.
Depending on what SW license you are using you can then set the blocks as ‘rigid’ which will treat them as infinitely stiff making convergence easier/quicker.
I did something similar a few months ago for my board but couldn’t get a good material to use. What are you using?
Again, depending on your license you might have access to more advanced fixings like virtual wall and remote torque. I wonder if the virtual wall on one truck axle would allow the trucks to get closer together preventing elongation.
At the end of the day, it’s only a simulation, nothing compares to real world testing! Try not to get too sucked into it!