Running the AM14U5 with the mini output shafts will give you a wheelbase of 20.00". As noted above, you will want your track width to be a fair bit larger than the wheelbase for traction wheels so you can turn. If you cut your front and rear plates about 27" or 27.5" long (max to keep frame perimeter under 120") you should be OK. Reference:
Note that with a 20" spacing, these wheels may be easier to connect with (inch-based) chain than with (metric) belt.
3946 ran 4 traction wheels a couple of times, but we had a much shorter wheelbase; we cut the back third of the KoP chassis and had a wheelbase of less than 12". Those worked great!