Hey, since when do we follow the instructions literally?
I have both those kits, too - but I didn't get around to building neither, yet...
But I think I'd complete the frame first with all the hardware cought in between the rails (the "dogbones" are not!), then I'd probably add the camelback springs (maybe without glueing them first) and then the axles with the short shaft between them, and then the "dogbones" last.
Anyhow - good luck with your gun truck and I'm lookin' forward to seeing some pictures! Have a nice day
Paweł