Not to be a buster or anything, but, when it comes to internal engine torque specs. I am more apt to be as precise as possible. Now, something like mounting bolts for A/C, or front cover, pump, etc. I would have a lesser issue doing by feel, or even guessing.
Manufacturer specified torque for main caps is 129 ft. lbs. With +/- 10% would make the range from 116 up to 142 ft. lbs. and at +/-15%, from 109 up to 148 ft. lbs. I would be very uncomfortable with that big a discrepancy on my engine's main caps.
The length from the pivot point is the main effect on torque, so straight extensions, albeit, the shorter the better, affect it minimally, depending on the material of the extension, due to flex, and as long as they are held as close to 90* as possible.
However, universals and / or swivels have a serious effect on and make for very inconsistent torques, at least in my experience. I think it can be exponentially worse, considering the torque sequence recommended is in 3 stages, 45, 88, 129.
There are several reasons;
1. the actual "twist" in the swivel, the wrench will still click where you set it, however an amount of that torque will not be applied to the bolt / nut. So the wrench clicks at 129, but torque applied to the bolt / nut is closer to 110.
2. how steep the angle is may actually multiply the torque that the wrench "senses", thereby reducing the actual torque applied to the bolt / nut. (same affect as above).
3. length of distance between swivel and bolt if placed at end of wrench before any extension, or wrench if placed at the end of any extension, can place the pivot a significant distance from the wrench's calibrated point. Thereby multiplying the torque applied to the bolt. So the wrench clicks at 129, but torque at the bolt / nut could be closer to 150.
Now if we figure in the +/- 10% allowed tolerance, that can add up to a MetricCrapTon of difference.
After all that being said... you probably won't have any problems. The potential is there, and I wouldn't be doing it, unless I was unloading the truck.
But I fall back on the old saying, just because something is made, sold, used or done all the time, doesn't make it correct nor necessary.
THE ABOVE IS ENTIRELY MY OPINION. BASED ON MY EXPERIENCE, KNOWLEDGE AND INFORMATION I HAVE ACQUIRED.
We're going to have to agree to disagree. There is no loss of torque from a twisting extension. Whatever torque you put in on one end is what comes out the other end. The shaft will twist and as long as it doesn't break at the point it stops twisting it then transfers the torque through to the other end. Google it. I did.
The u-joint will cause a plus, then a minus reading, but is very much dependant on the angle. What I've read says that if you have under 15* of angle you simply disregard it all together. There is even plenty of argument on the engineering forums about even that. Of course if you try to go through too sharp an angle it will bind up, but that's something different all together. And that would be in a cardon style universal. If you are using the pin and ball type used with impact sockets the change is even less.
Cummins is a little more specific about their torque specs. Most engine manufacturers you will see it written as something like 95-105. On those if you take the centre you will be perfectly fine.
I would always attempt to not use a universal and adding extensions with flex, while not making it less accurate, make it more work. You have to pull your torque wrench a lot further through this spongy area. I will use a 3/4" drive extension reduced to 1/2 if I have to use an extension for that reason.
I've been a journeyman doing this for a little short of 40 years and have never had an issue where I've had to use a swivel. And I use extensions all the time.
On a different, but related point, Cummins does some things a quite different, and I have to wonder why. I have never seen an engine that req'd the mains to be tightened in stages and in sequence (other than snugging them all down to ensure they're seated). What is the point of that. The caps fit flush against the block. The only crush is the slight bearing crush to seat the shells. It's not like a head gasket or an intake where clamping force needs to be even and force the gasket into position. It doesn't matter how much you tighten a main, it doesn't get any closer to the crank. The bolt torque has only to do with removing the stretch and reaching the point it won't come loose. Block distortion during tightening? You wouldn't think so in so heavy a block. I do lots of diesels, mostly ag and have done gas engines forever and have never come across that. Engines that use torque to yield bolts will do stages, but that it just to get you to the point of bolt stretch.