I got the new 3 in 1 251 kit from DML the other day
and chose to do the version w/ the tank gun mounted.
Step 1

of the instructions focuses on parts that fit on the inside of the lower chassis and a couple of suspension parts. The gas tank and other parts realy don't need to be made and glued in as they won't be seen in any case once the floor goes on. But I made mine anyway. - didn't bother topaint them though

I choose to depart from the instructions and start with the suspension as the one assembly at the back needs to tie-in with the axle parts . The instructions are not clear about this but hopefully I can help clarify this for you.
So gluing the three parts together I then added the axle arms. There is a pin to help align things but to make sure the wheels all would be level later on I jury-rigged a jig using some I beam plastic strip
and solid rectangular stock. the I beam keeps everything in line on top as the rectangular stoch gently puts pressure underneath. The fit is just tight enough for things to line up. The I beam stuff is CA glued to a glass work surface- so it can be removed later and reused.
the pennies add weight to the middle section to keep it flush with the glass surface.