I agree that it's an unusual find. It's actually curious to think about. If the engine was installed used, that means someone managed to find an original '57 truck engine, then did the work to pull the six and install the eight without painting or rebuilding it, then left it alone for years on end despite repainting the truck. The odds against finding an original v8 engine seem high enough without thinking about the work required to swap it for the six. It would make more sense that the swap was done years ago when the '57 283 was closer to new. But still...
The six cylinder front mount was attached directly to the frame while the V8 mount was attached to brackets that are riveted to the rear of the crossmember. Until a fairly recently the V8 mounts were not available as reproduction items so someone wanting to accurately swap in a V8 had to cut the mounts out of another frame. The V8 fan and waterpump sit further back in the chassis than the six so GM installed a separate radiator support to position the radiator closer to the fan. Attached to the top of the core supports is a flat plate which is primarily cosmetic. Like the V8 mounts, these parts were not available as reproduction items until fairly recently. The wiring harness for the V8 is also different. With the coil at the rear of the eight, the wiring to the coil is much shorter than the wiring to the six cylinder coil. The generator is on the top driver's side of the eight vs halfway up the side of the block on the six, resulting in shorter wiring. Of course all of the wiring can be adjusted but its usually easy to tell the difference between OEM wiring and repairs or modifications performed in the field. And then there's the foot pedal starter. AFAIK the V8 trucks were equipped with a full ignition switch while the six cylinders with manual trans still received the foot pedal starter. I believe the hole in the cab toe plate for the starter pushrod was a permanent modification, IOW V8 trucks did not get cut. There's also the issue of different throttle pedal linkage, different hand throttle and choke cable lengths, etc.
If your truck has riveted engine mounts, has all the correct parts for the v8 swap, has an apparently original harness, then I'd really be asking how this combination exists. What are the odds, do you think, that someone swapped the cab or VIN for some reason? It seems far easier to do than locate the V8 then swap it over. If there's no hole in the floor for the starter rod I'd really have to think hard about the constitution of the person who might have made the swap. Making so many changes look like they were never changed would require a *lot* of extra effort, and it's rare to find that level of work invested in a truck which is subsequently treated as only slightly above average.
There is an online version of the factory assembly manual here:
https://www.trifive.com/55-59Assy.pdf
If curious, you can use it to check originality of the engine installation.