Rear Axle and Axle Shaft Help

[drbeeper17]

I just started working on my project after it collected dust for 2 years and I have some questions about the rear axle. The truck is a hybrid of parts ranging from 67-72, which is why I do not know exactly what I have. My question is how to definitively tell if I have a narrow or wide 1/2 ton rear axle. It is currently setup with 5 lug axle shafts, but I have a strong feeling that parts have been swapped with it. It could be a stock 5 lug axle with all stock parts (which would be ideal) OR it could be a narrow axle originally equipped with 6 lugs converted to 5 lugs by swapping properly sized shafts (the lugs have not been re-drilled on the axle shaft) OR it could be a narrow axle originally equipped with 6 lugs with some combination of shafts and drums to convert to 5 lug.
I've read posts for about 3 weeks now and I can't determine where everyone is taking measurements from. I keep seeing that backing plate to backing plate of a narrow rear axle is 55.5" and the wide axle is 57". But depending on where on the backing plate I measure, I can get either dimension. The backing plate contours, so if I measure where it attaches to the axle flange, I get a wider measurement, but if I measure from the outside edge, I get a narrower measurement. Additionally, if I measure from the inside edge of the backing plate vs the outside edge of the backing plate, I get different measurements. With the axle widths differing by only 1.5", it's really hard to tell what I have. I want to rule out any measurements involving the axle shafts or drums since the ones on my truck may not be the correct ones for the truck. If I totally strip the axle down including removing the backing plates, is there a measurement for tip of axle tube to tip of axle tube? What about the width from the inner or outer sides of the backing plate flanges?

For additional background, I do not know the year of the frame, and the rear axle may have been swapped at some point. I do have 5 lugs front and rear, but the front has been changed to an all new 5 lug disc brake setup prior to me taking possession of it. This makes me think the rear was swapped as well to match the 5 lug front. The frame and axle have been painted in a thick black paint (also prior to me taking possession) and the numbers on both do not come up with any definitive results. I think the frame may be a 69 or 70, which means the rear end may be narrow or wide depending if it is a 70. The radiator mounting holes are about 1.75" and the bracket sits flush with the frame (making it a 69-72 frame I think). The front brake lines route through brackets attaching to the rear of the front cross member (making it a 69-70 I think).

Aside from providing some definitive measurements from definitive locations on the rear axle, I also have some pictures of what the axle shafts look like sticking out. Are they supposed to stick out this far? Since the axle shafts do not look aftermarket, my concern is that someone stuck longer shafts into a narrow axle and called it a day. I plan on running around 500hp, so I don't want to have a rear end that is not set up properly. The axle shafts measure 31.25" from inner tip to the flange where the brake drum sits (where the threaded portion of the wheel stud comes through the axle flange). This is once again a hard to know what shaft this is since some measurements I have found measure from the end of the splines and the shafts extend beyond that for the C-clip retainers.
Attached are some pictures. Hopefully someone can tell me what I have based on these pictures and the descriptions above. Thanks!!