Hi Tom, welcome to the forum.
In my opinion it is much easier to assemble as far as you can and then paint. For example, assemble the chassis with all its assemblies and accessories and then paint. Of course chrome parts sush as fuel tanks toolboxes etc, should be fitted later. Then assemble the engine, then the cab.
Don't forget to take your time with your models, its not a race, you doing it for plesure and join.
Remember always test fit, test fit, test fit, then test fit and then glue the parts together.
Kostas.