I think a few of the early 406s were not crossbolted, eventually leading to them being crossbolted until the 427s came into being. The "aftermarket" installation of the crossbolt systems were/are targeted towards towards the 428s and 390s, mostly. To my knowledge the only difference between the side and top oilers was the oiling system, to include block casting variances to allow for the
oil galleries. None of the variances included strengthening of the side oiler block over the top oiler though..
Dave