There’s a guy on shapeways (gmphnd) that makes the hood, bumper, headlights and grille to convert the Italeri 378 to the Optimus Prime Peterbilt 389. I’m sure you’d also have to extend the frame. To be more accurate, he also has the cab and sleeper you’d need. I found it by searching for 1/24 Peterbilt. Shapeways search function leaves a lot to be desired, so here’s a link to the grille:
https://www.shapeways.com/product/WCNLTCMEU/1-24-optimus-prime-grill-part-italeri-peterbilt?optionId=61182443&li=marketplace
The parts are not cheap, and they certainly may come with the downside of 3D printing, so I’m not sure how smooth of a finish you’d get. However, you may be ok with the more expensive (smoother) materials, along with sanding and filler primer. I don’t have any experience with 3D parts myself, except for small detail parts. Some guys have used them to make resin masters, and had successful results making good resin parts in the end.