Fingers crossed it isn't a rear main seal. I'd be a bit worried about obvious smoke coming out from under the cab. The clutch may have already been toast. This truck has had no love for a long time and with all the grief you've had with it, I wouldn't be surprised if it was worn out part it's serviceable life. If you have clutch slip, you can be pretty sure it's tired - it'll be exaggerated by a ton of new found torque and power being pushed through it. You shouldn't have passed the threshold of clamping pressure from the clutch assembly yet. Don't forget that the brakes haven't been worked for ages and the smell might be from them.