Those inline 6-cyls are near bulit proof… HOWEVER they are known to blow there head gaskets right around 200-250,000 miles…LOL, I know it seems high mileage to have a known issue, but many of them hit this mark… So many infact that the head gasket issue is known. This would explain your low compression as well.
Less likley, You could also have damaged the carb so bad it is flooding the motor and that is why you will not start… Try this… Remove the fuel line from the carb and put it into a fuel jug, spray starting fluid into the carb… Will the truck fire now??