[RichardJ]

37 turns? That doesn't give me any more confidence than your previous 15 turns.

Jack the axle so that both tires are off the ground and place stands under the axle for safety. Chalk or tape both tires on the inside and the concrete as well. With a mark on the driveshaft, turn it by hand.
As long as the brakes are not dragging, both tires will turn the same amount whether it has posi or not. The tires need to revolve 1(one) turn only. The driveshaft needs to be turned between 3 and 4 turns only. 3 and 3/4 turns for the 3.73:1

As already ask, what is the tire diameter.

My truck used to be stuck in the slow lane at 60 mph, on the level or hills. With 4.56 gears the engine was screaming at 60 mph.

The change to 3.54 gears moved the truck to the fast lane with an easy cruise speed at 75 when traffic permits. I DO NOT tailgate.

So yes, more suitable gears will make a truck go faster.

I this case however, it seems the 250 can't pull the OD 5th gear on the level nor the 4th gear on the hills.

The 250 shouldn't be down all that much power from the 292. Until I hear a more confident gear ratio and the tire diameter, I'm leaning toward the 250 being down on power.

Because the collectors are staggered about 4", it does make it more difficult to make an "H" pipe that exits and enters the flow at 90*. Both ends of the "H" pipe are the same distance from the collector. The mufflers with bottom in, top out are staggered the same amount.
Don't expect much if any performance improvement from a cross-over. It will help eliminate that nasty inline six raspy sound.