x2 on that.

Hmmm...
I was thinking shaker screen bolts were just grade 5 carriage bolts. LOL :)
Ooops....

Shaker Screen Bolts (Full threaded carriage bolts) on National Bolt & Nut Corp.

Carriage Bolts on National Bolt & Nut Corp.

Funny but I had one Phillips+button head bolt over my mid-ship tank.
My F100SC parts truck has two in front for similar reasons.

Button Head Bolts | Portland Bolt

Ford didn't use the funny looking extra longer square "truck bed bolts(?)"
every time either. So neither do we? ;)

Do you still need length measurements?
It's 80F+ out there right now. LOL :)
January and February are the two nicest months around here.

Alvin in AZ
ps- It was easier for the factory to put the Phillips head bolt where they
did on my '75 F150 with the mid-ship tank... but I moved it to an easier
spot to get to because I want all carriage bolts and will be robbing one
off my parts truck if any are the right length.

 

Hey Alvin could you possibly box up 20 degrees or so and UPS them to me?

I used shaker screen bolts on the last dentside I had. Overkill is true but they were freebees.

 

Gladly, the sun peaked out a for a few minutes while I was collecting tools
and laying down the sheet of cardboard from Costco... it was trying to
friggin cook me because I had a black tee shirt on.

Used to, you could spot the "fernurs" by their dark colored vehicles. LOL :)

8 foot bed...
The six rear carriage bolts are 3+1/2" long and the two front ones are
4+1/2" long. That's just the threaded part, didn't include the dog nose
on the two front ones them being Phillips head and all.

1/2-13 bolts (3/4" wrench)

I used a #4 Phillips bit but might could use one size bigger?

-----------------------------------

Here's a trick I learned while doing motorcycle work...
I like Phillips head screws! :)
Some guys go out of their way and buy a set of Allen screws.
Strip the head on one of those and out comes the Dremel to grind a slot
into the head.

Anyway...
Hammer the Phillips head down so the screwdriver won't fit into the hole.
Then... hammer the screwdriver into the hole.
Perfect fit, see? :)
...and by the time you get done with all the hammering...
many times the screw is about to fall out on it's own.

YMMV

I use carburetor cleaner then spray some Liquid wrench (diesel fuel) on it.
The guys at the wreckin' yards like to use straight antifreeze, never tried it.
They have to deal with that rusty stuff "from back east" tho, see? ;)

Alvin in AZ

 

You are measuring a 1975, the OP's truck is a 1969, the bed bolts are not the same.

 

If the bolts are the same with a 250 as the 100, then you should be able to use 1/2" instead of 7/16". the 7/16 dont have a large enough head on them and the 1/2 should fit (they did on mine at least) buy em long and cut to your need, the 2 bolts near the cab are longer than near the tail