On an 8 1/4" rear, the axle centerline to pinion yoke centerline measures 11.69".

On an 8 3/4 rear, the same measurement is 12.35

That's a little over 5/8". If the suspension travel on my B200 can tolerate that same swap I bet a Road Runner would be fine too.

'course the RR is worth at least 10 times as much as Junkyard Jitters, my ratty old '77 Tradesman, so the decision is still yours.