Solid-modeling: Old school style.  I'm finally close to getting the seating position "just right" on attempt #3, while also making sure the engine/transmission will sit far enough back in the chassis and without making the chassis any wider so that it will still fit between the rear wheels.  This would be a lot easier if two objects could occupy the same space...So really it's just the age old problem of fitting 10 gallons of poop into a 5 gallon bucket.