What is the cranking speed with the spark plugs removed? It should spin noticeably faster with the compression relieved. If not, there's too much drag somewhere- - - -rusty cylinder walls, tight bearings, too-thick oil, etc. What temperature is the engine? If the truck is parked outside in very cold weather, it might be necessary to do a 12 volt jump start, but I've seen lots of 6V vehicles start reliably even in sub-zero weather. Did you load test that "new" battery? It should deliver 250-300 amps for 15 seconds without dropping the terminal voltage below 4.5 volts.
Jerry
