This is going to require some math on your part. It's not his fault, as he made the file at 1/10th scale, so asking him to do math for you is a little outside the norm of him providing the finished file.

Here's how I would do it, and I'm sure there are other ways, and it's a bit more complicated because you'll have to do this each time, for every single file that's provided there, but take one of the pieces that exists, and plug it into your slicer.

Take one of the dimensions, let's say the X size value, and let's say it's 100mm in Length. Using a calculator (there's an app for that) take that 100 and divide by .85 (which is the decimal representation of the scale 1/8.5 (that the author is referencing to give you the proper wheelbase you're after). That gives you a new number, 117.64. That's your length of the 100mm part at 1/8.5th scale.

Now that you've got that, that's actually your scaling factor that can be applied to everything you need to print, because the math I gave you is also the way you calculate the percentage to scale by. Neat huh? And I'm terrible at math!

Now for the bad news, this is going to make this truck HUGE. Like, won't fit easily on most build plates. And for the double bad news, you sharing this file around for others to look at is probably against the terms of service for Cults3d. So I've deleted the link from your post. Because that's pirating, and it's not something I can condone here. Good luck with your print.