assume the truck ran after the transmission install, but has otherwise sat for an extended period without running. And no other changes have been made or work performed/parts swapped since that time. As noted fuel pressure would be number one to check (appears in this thread that has occurred). One also assume you have good battery or batteries and no issues with loose cables or connections and engine turns over well but no start. Also assume you have no trouble codes set in the ECM? What if any things have occurred since the last time the truck was running? Does the truck need the neutral safety switch active to start?