Recommended 8/34

The 489 has the largest pinion at 1-7/8" vs. 1-3/4" for the 742. The 489 replaced the 742 starting in the 1969 model year. The 741 is the weakest of the three and I think it goes back to at least the early 60's. I think the typical 741 used the smallest u-joint also.

If it was me, I'd use the 742 simply because the sure grip unit is more easily re-buildable as it uses replaceable clutch discs. The 489 uses a cone-type clutch that eventually wears out thereby losing its locking ability so you're back to a one legger. I know someone on here rebuilds the 489 by machining and using shims, etc. But if you have both on hand, I'd go 742 just because the clutch plates are still available (try Mancini's for instance).

The B-body 8-3/4 is definitely wider than an A-body axle. And there were different width B-bod rears depending on the vintage. You can make up the difference by re-welding the perches and carefully choosing the rear wheel backspace. I know some guys just spread the springs apart where they meet the perches but I personally wouldn't do it. If you already have brakes with the B-bod then you are good to go (or stop as it were). IIRC they originally used a B-body rear in the 68 SS Hemi A-bodies with the offset spring kit. You can always sell your a-body rear to help pay for some of this.

Or use the A-bod and replace the axles. I had C-body axles shortened and splined by Mosers for approx $100 plus the 35 I originally paid for them. But then you have to re-do the rear brakes. I used backing plates, drums, hardware from a B-body (10" x 2-1/2")

The various center sections swap into the housings no problem. 741-742-489. I think you mentioned using SBP to BBP wheel adaptors when switching to racing tires which are going to exert a greater force on everything? Uh....no.

The wheel selection in BBP is much greater than what is available in SBP.
If it was me I would convert to the 4-1/2" pattern now so you don't have to screw around with adaptors, stacks of SBP, BBP wheels, etc. Just do it once and don't look back.

Oh yeah, at one time the factory recommended the 3.55 ratio as being the best match for a 340 on a new car but that's your call.