For a motor mount you might want to consider something like this
that attaches to the center diff mount with the brake disc screws. With thick enough material you won't need a brace on the chassis like my example. But if you make your own chassis adapting a 1/8 diff and motor mount would be easier than with the stock chassis.
For a body, the kyosho DRT body is the closest in size, but the wheelbase is still 23mm longer on that truck, which with some careful trimming should still look good. And the width is alot closer DRT=257mm, Tomahawk=245 than the alternatives. Since you plan to go 40mm longer I assume you're thinking slash body which has a track width of 296mm. Not trying to talk you out of the extended chassis, just providing some info. Looks like a cool project, keep us updated.