Yep you use the old bolts for clearance check with the plastigauge. It's also commonly recommended to include old bolts with the rods when having the reconditioned. Apparently the clearances check out OK but the bolt strength is insufficient with used bolts? Not sure on that.
I do know that pros have advised with ARP bolts that the rods should be resized with the new ARP bolts.
1) using the bolts burnishes/wears them in and makes torquing them more consisent. That said, the burnishing technically is not an issue when you use their special lube (how convenient.....
)
2) since ARP bolts are torqued to a higher specified amount, the additional load distorts the shape of the big end slightly and therefore the rod should be re-sized accordingly. There's one more reason not to switch to ARP rod bolts w/out disassembling the case.