I recall from another forum that Mark Larue strongly advised against bridging the receiver with one of his mounts. Seeing that you have gotten the same dope from both JP and Larue I would strongly consider looking at other options such as changing your stockweld or trying the ARMS mount with rings as previously suggested.
Another option, albeit not easy or inexpensive would be to consider either a Vltor VIS type upper receiver/rail or the PRI Gen III tube with Rifle Length top rail. This rail mounts to the top of receiver and extends on the tube. Either of these setups would allow you to use the Larue mount with the desired relief however the PRI setup may be too high (and heavy).
Good luck.