A Ford "Highboy" is an F250 made from about 1973 to about half way through 1977 with a narrower frame, higher suspension, so-called 'divorced' transfer case (short drive shaft between the trans and the transfer case), narrower (2.5 inch wide) rear leaf springs, gas tank in cab, etc etc.

I can't say why the Ford 300 inline six is so good except that it is obviously 'understressed' with 7 large main bearing journals and very strong cast iron (alloy) block. The long stroke (about 4 inches) gives excellent torque but probably is not ideal for high revs.
In general, straight sixes are very easy to balance, especially compared to V6s that apparently are difficult. If you think about some of the smoothest engines in BMWs & Mercedes, the old Jags & Maseratis and the recent Japanese luxury models, they are straight sixes.
I'm sure there are many important engineering advantages to the good old iron six but many people have simply been impressed by the durability and low maintenance..... so I like the 300 I6 the best in a workhorse Ford pickup.