Custom lowboy 'kit' parts needed...

[JensR]

Not sure what level of detail you want and what degree of manufacturing you can do, but...

Smaller than Tamiya wheels are offered by several companies, but the easiest to get in North America might be the Wedico tires. They are 1:16 and hence just that that bit smaller to look good on a lowloader (IMO).

Axles can be modified to Tamiya width from the tire-matching Wedico axles or scratchbuilt completely.
If you want steered axles, again there are some manufacturers out there. I know of SchulzTec in Germany. But they come at a price.
OTOH, scratchbuilding a steered trailer axle is also not that difficult.

Main frame rails can be made from aluminium profile at your local DIY store.

Swing out wideners are sold by Veroma and SchulzTec.

As for heavy duty rims for the tractor unit, have you checked the RC4WD ones?