No, the 68310's won't fit, at least not without a lot of grief. I went down this rabbit hole with my own Project and have posted pictures in a series of threads tagged Project Regress. Between my speed shop and my own efforts I was able to learn that the Hedmans will fit only if the front motor mounts are changed. The frame portion is okay but the clamshell interferes heavily with the driver's side pipe. As noted previously, UMI does make mounts that allow both the Hedmans and Flowmasters to fit but...…..
The but here is those mounts are only the first issue. You might get lucky with the hedmans once you get past the front mounts but you could also find that the tubes on the driver's side have another problem. Specifically Number five crashes into the front pair of mounting tabs for the lower control arm and slightly less at the back pair. The Flowmasters are no better, in fact, my Flowmasters now have a serious dimple in the number five tube just to deal with that need for clearance. The dumb thing is that Hedman will not include any type of disclaimer concerning this problem. I only found out about it when I visited the OPGi website and took a peek at a header sale they were running and there it was, a list of what fits what. What compounds the whole thing is that, today, I slipped under the Monte to install the starter motor and figured that I would probably have to back off the mounting bolts to get the header to move enough to ease the starter past it like I had to do with the Hedmans I've run in the past. Not necessary. The starter eased into place like it was made to do it. Super stupid part of all this is that the interference issue could be solved by exchanging the routes of two of the pipes as they make their way to the collector. Same foot print for all the pipes, just exchange how they lay. Why or how the engineers missed this one is beyond me unless this is a farm job and the work is getting done somewhere else. It would be nice to correct the problem but I don't Tig and these pipes are both S/S and ceramic coated. The TIG can be learned but that coating could cause me a world of grief if it isn't removed down to bare metal and then how do I get it replaced?
Good Luck with your search and if you are successful would you post the part number and MFGR to add to the local data base?
Nick