I know this is an old thread, but I'm dealing with the same issue. My engine had two hex bolts that went thru the front engine plate and into the rubber mount. One had the head ground off at an angle so it would fit in place with the timing cover and balancer in place, and was 1-3/4" long while the other was 2" long. But dummy me figured that they needed to be carriage bolts and cleaned up the square holes in the front engine plate and now even that "customized" hex bolt won't go in. I don't think carriage bolts will fit (but will probably try, as their heads are shorter than hex bolts.
In researching in the FAM, I saw that there's a note on those bolts thru the front engine plate that says "furnished by engine plant" So my suspicion is that the bolts were swedged in place in the engine plate BEFORE it was installed. Meaning that I won't be able to get a bolt of the proper length thru on the right side, even with a carriage head.
I'm going to pick up some appropriately sized 7/16 dia carriage bolts and give it a try. I will post what I find out.
