Just talked to a guy at Dart and got one of the part numbers for the 4.125 bore option and a price. A bit higher than I thought. $5200 and part number is #31937211. One thing to consider is a special oil pan has to be used from Milodon or Canton. A standard LS oil pan will only work with an adapter. A few other things also, but I forgot....LOL!!
I think I'll stick with the RHS due to the price.