I tightened my CE forged perch bolts with Gr8 nuts to 220 ft-lbs and the shock mounts stayed in place. I suspect some other brands of parts may not like that.
I've even put a shim between the axle and the wishbone/batwing to make sure everything tightened up solidly.
I believe the thread is a 5/8-18 NF thread. Material equivalent to grade 8 bolt? chart below may give you a good starting point.
240 ft lbs seems like alot. Ill just get it as right as incan and maybe double nut it if possible. Thats a good idea, i may do the same to keep from moving
If anyone is buying perch and shock mounts, you might check these as they do have a keyway in perch bolt and key in the lower shock mount. Roadster Supply Company https://www.roadstersupply.com/searchresults.asp?Search=Perch&Submit=