I can't address the quality of the Fastenal threaded rod. Rolled threads can have denser and smoother surfaces than cut ones. Torque is just an inexpensive way to measure tension in the fastener (clamping force). Torque is also a function of rotational friction so smoother threads and nut and washer faces are better. Due to the physics of inclined planes fine threads on one end of the stud allows better control of clamping force versus torque. One advantage of studs (or threaded rod and nuts) is that they can be fully engaged into the block before they have any tension on them. This makes the torsional friction a function of the threads and nuts instead of the threads and the cast iron threads in the block. It also preserves the threads in the block and vastly reduces the error induced by the twisting of the fastener even with coarse threads on both ends.
From what I have read the stock bolts are designed as single use bolts (torque to yield on initial assembly?)
Keep the rod cool when you cut it and clean up the ends so you don't alter the tempering. You could do some testing with a short length of rod through a steel plate with nuts and washers on both ends to get a feel of how tough the Fastenal rod is.
I've never had a head off a Cummins but chances are that a bottom tap run into the block could let you run the rod into the block a few more turns. This may not be necessary but if I was going to this trouble I'd feel better for doing it.
I agree with everything being said here except that bit about inclined planes. The thread is 60 degrees in both the course and fine, vector is the same in both cases. The advantage of fine thread is they can support higher clamping forces because of increased surface area, but only about 10%. The thread engagement on the top is limited by the nut, that's why they're fine thread. More threads in given distance.
1997 XCLB 12v Cummins 4wd. Goerend Trans: 1800 Stall Triple Disk, Billet Input, Internals, Valvebody. Pillar Boost, Fuel PSI, Pyro. DAP 5x12 SAC. 4" Diamond Eye TBE. Wix BHAF w/ outerwears. HE351CW No Spring Gate