LMC,Jim Carter,Classic Truck parts,just to name a few,all sell replacement cab seals and grommets,including the steering column. If your exhaust runs all the way to the back beyond the bumper,I should think that you would have N0 fumes in your truck. Do you have a sliding rear window?? that will draw fumes way more than a side window. Stuff a couple of "Brillo" pads,in the exhaust ends. They're steel wool,and will constrict the exhaust flow,without stalling from back-pressure. Dump a little transmission fluid into the carburetor throat,while having someone control the throttle. This will smoke like a mosquito fogger,but it helps clean valves,too. If you have any leaks in the exhaust,this is the best way I've found to see any,that aren't otherwise obvious. hope this helps.
