To answer you question to the best of my memory ability....from all the FTE Highboy threads.....IOM
1. Look for a cross member between the front frame horns, below the front bumper
2. Front spring pack will have 6 or 8 leaves?
3. Back springs will be 2 1/4 or 2 1/2 wide?
4. A ram style steering cylinder on the front axle/tie rod end w/ p/s lines
5. Steering g/b on frame will have p/s lines going to it
6. Behind the seat tank or saddle bag tank, cause frame is narrower in rear, BIG maybe on this one.
7. Divorced xfer case (not direct attached to trannie)
8. Front/rear drive shafts are close to same length
'66 marked the first year of the Highboy in the F250 line. They ran up to '77.5 in which midway thru the year Ford changed the 3/4 ton line and went to the "Lowboy". VIN numbers determine whether a '77 is that or the '77.5 year.
The Lowboys after 1977.5 will have married transfer cases which are bolted right behind the transmission via an adapter. Front driveshafts are shorter and rears are longer. Also came with hi-pinion diffs, leaf springs are flatter than the Highboys and frame bends are different, too.
'66 began the Highboy and it ran clear up to '77. I know I mentioned '66 where the '67 began the body change. The '66 is the old body before the '67 started.
The aftermarket line of suspension lifts is available for the Highboy from Skyjacker. They offer 2" and 4" lifts, both with front springs and the rear, narrow springs available in 4" only.
Highboys have 3" wide front springs and 2.25" wide rear springs. Lowboys after 1977.5 have 3" wide front and rear springs.
Where a 4" Skyjacker front spring lifts a Highboy that much, it will lift a Lowboy 6". The Highboy has a difference of 2" of raised height than the Lowboy. Rear factory blocks in Lowboys are 3" and Highboys are 4". These springs don't lift the highboy an additional 4" they are basically replacement springs. I know I have them on my truck.
There are some confusing ideas on what makes a truck a highboy or not. First of all the term highboy was first coined by the owners of the trucks according to a Ford parts man I know, he said that Ford made, nor assigned no name or title to these trucks. What makes a highboy is that the truck is assembled using different springs etc... to change the ride height. I don't know if this applies to the '66 thru '72 trucks, so here goes, I have had two (2) Ford trucks 1976 & 1974, both F250 4WD both had divorced transfer cases. One of them sat approx. 4" higher than the other. Also, when ordering parts for the trucks I noticed that there was a split in the VIN numbers for a lot of the parts. Trucks with VIN's before #Y20001 sat higher than the trucks after #Y20001, now this is just an observation of mine. While my trucks were both stock, the 1974 sat lower and had front springs that were almost flat and had no blocks in the rear. While the 1976 had front springs that were arched quite a bit (4") and the back suspension had blocks with a bump stop arm cast in them they measured (4.25") high.