Congradulations on the new rig, and welcome to the world of continously fixing things & making improvments on these 40+ year old classic's.
It is very common to have exhaust fumes in the cab, many Bronco's are so bad that it'll make your eye's water & stomach nauseous.
Often times its a poor seal on the rear door and/or tailgate. First, fix any & all obviously exhaust leaks, make sure the tailpipe exit's in such a manor to creat a low presure vacum/venturie' so the exhaust gas's and fumes can fully exit and clear the Bronco. Such as turn out/down tips, and/or route the tailpipe out the back corner's...NOT out the back under the bumper.
"Fixing" the rear of a Brocno to truly seal nice is a whole can-of-works in itself. Making sure the door is aligned perfectly, making sure the tailgate is aligned perfectly, the top, and good quality -well fitting seals. This may sound simple enough, but my expericne has been that this can, and often times does take on a life of its' own. ....fix one thing, then discover 3 more!