Those engine mounts are '73-'87 style. I can't tell from the pics if the frame brackets are from a C10 or a C20/30. If they are from a C10, that is part of the problem. Those do not fit the early frames correctly without a spacer on top of the frame. Without changing the frame brackets, you will have to search and try and find a set of clamshells that are shorter. That or a different set of Ls engine mount plates like these.
http://www.ebay.com/itm/Dirty-Dingo-...dW3aG-&vxp=mtr
I do not know if these will cure your issue, but they might be worth inquiring about. AZ Pro Performance is a forum Vendor here.
The more I look at those pics the more I am now fairly certain those are from a C10 and they put them on the wrong side, slid them down and drilled new holes.