[WorkinLonghorn]

I've always heard that these trucks got the 4 bolt mains.

Also, this isn't what you were asking but a Longhorn is a heavy truck with the One Ton frame. My '69 with all options and a few tools weighs 5300 Lbs. This is not the ideal platform to make a quick or sporty truck in my opinion. I had a stripped down 1/2 ton with a modified engine that would hold it's own against most cars (back in the 90's).
Sometimes I wish for more power in my Longhorn but I usually think about TORQUE, maybe a torque 383 build.
Honestly though, if your engine is that low mileage, I would check the vacuum, compression, ignition advance settings, carb function and make a few changes like jetting, cold-air intake (not the open element crap that sucks in hot air from the radiator!) as a start. -BA