Looking at D150 terrible spark knock

Check the intake for the presence of a working EGR valve. If you disable it, then there is no way for the Exhaust to enter the intake, even with the floor-jets removed.
To prove the EGR valve is closing. Just stick your fingers underneath diaphragm, at idle, and lift the diaphragm up. The engine should stall or nearly so. If it makes no difference then the circuit is not working, unless the engine is already idling like crap, then the valve may be stuck open, which I have only ever seen once,lol.

I have an 84 here, and only a few years ago it was a great truck.
Try and get a compression test done before you buy it.... if the pressure is over 165psi, getting rid of the detonation could get expensive.
Buy it for the body, and do whatever it it takes to get rid of the detonation.