[B]what the guy said is partially true, yes originally due to the increased weight and restrictive emmissions controls it was easier to build a bigger engine so the increased torque would get the land barges rolling with those numerically low rear gear ratios, That being said a 400 will probably be a great engine for your truck and with the help of some of these guys on this forum I am sure you will be pleased with the outcome.
I have learned so much by just asking and paying attention to our resident experts.
good luck with the truck