Centering the engine

I'm planning to move my engine back 18 inches and I'm wondering if there is a good reason not to "center" the engine, since seating interference would no longer be an issue.
A little measuring tells me that the frame rails are not offset, but that the stock engine cradle is.....I can't see a difference in leaf springs, side-to-side, and wonder if the right side engine bias was meant to be compensated by the weight of the driver (?)

I believe the motor is off set to match up to the off set with the rear dif.

I had a 69 Ford van years ago that definitely had the pumpkin offset to the right to match engine placement.....I don't think Chevy and GMC have offset pumpkins (guess I'll need to measure)

The early Chevy vans' rear is offset. Unless you are swapping the rear in from another application, you'll need to maintain that engine offset.

I ran a tape measure on it and the driveshaft yoke is exactly centered between the left and right leaf spring and the diff appears to be stock......
I HAVE seen GM differentials on center where one axle is longer because of the extra distance into the carrier on the side that holds the ring gear (I think they are all this way, but I'm not sure) The 12 bolt in my 68 Suburban is like this....

If it is a van rear axle assembly the axles are the same length. It is the pinion that is offset in the housing. Some pics would be nice?

Leaf spring to the yoke dust collar is 21 5/8 inches on both sides and both the front pulley and trans hanger bolt appear to be one inch right of center (2" difference in measurement per side is corrected by taking away an inch on one side and adding it to the other)
I'm not sure that a 1" offset is worth the effort to correct or even if it results in a measurable side-to-side weight bias....The u-joints probably actually like the slight phasing.......