A 4.250 crank with 6.80 rods makes a nice combo. Diamond has shelf pistons for that combo. You'll just need to buy a new flywheel or flexplate with the 8 bolt crank.

Everyone should get rid of the 8 bolt crank option since the extra bolts aren't required but for some reason the mfgs keep making them. Most of the crank guys have gotten rid of the Mopar rod journal size so I guess that is at least a start.