Like my head studs I purchased stock QG18DE main bolts and had Greg measure them for the closest ARP equivalent. Picture below of the stock main bolts
Here's how the main bolt measure up. Pix courtesy of Greg.
I must be lucky as there are two possible ARP main stud options. Picture below summarizes the two options.
Per Greg, "the stock bolt threads 1.325" (33.655mm) into the engine block, 1.75" (44.45mm) inside the mains and only approximately 0.5" (12.75mm) of bolt head outside. The 4.125" (104.775mm) stud option has more threads at the bottom , assuming it bottoms out equally with the bolt, it has a shorter section of unthreaded partand you would tighten the nut down and there would be threads inside the main. Not the end of the world but there would be approximately 0.625" (16mm) left above the nut as exposed thread. I don't think there will be clearance issues but I'm not familiar with the QG18 motor (engine). The 3.75" (95.25mm) stud has less engagement thread in the block, though at 60 ft/lbs torque, 1" of thread is fine per ARP. This leaves you an equal to the bolt unthreaded section in the main and will only leave you with 0.5" (13mm) exposed"
Per Toby's advice I decided on the 4.125" (104.775mm) main stud option. Unlike me ARP head studs the main studs were in stock and duly arrived in about two weeks. Greg included a nice ARP box for me this time round.
Opening the box revealed the following contents - main studs, 12-point nuts, washers and a sachet of ARP fastener assembly lubricant. Nice!
Here's how my main studs compare with the stock main bolts.
These are 8740 chromoly alloy rather than the stronger ARP2000 as ARP doesn't have the stronger material for said main stud. Still loads stronger than the stock main bolts though. You can read up about how 8740 chromoly alloy measures up to ARP2000 here. I'll update what application the main studs were originally designed for when and if I get the info from Greg. Am still not done with ARP as I now have their stainless steel manifold studs on order.