Don't guess!!! Pull the belt(s) and then remove the thermostat crossover. Fire it up less than 2 min and see what side(s) has/have bubbles. Then you can look at that side(s) for the cause. If you don't find it in the heads or gaskets you are looking hard at the block. Note the timing cover can leak coolant into the oil.
IMO it's easier to pull the entire engine out complete with exhaust manifolds and turbo still attached, put the rattle gun to the stubborn manifold bolts and head bolts... Replace the tired rings with gapless and other assorted leaking gaskets ... (Really may only need fresh rings and possibly cam bearings, but no other bearings and just a light glaze breaking.) then put it back together on the stand. And the major reason to pull the engine is to get a good look at what gasket blew or cylinder/block crack.
The ARP instructions have changed over the years, but, the 6.5 head bolts go into "wet" holes that require a "Good" sealer. Good and ARP branded sealer don't go in the same sentence as sealer. Quick and dirty to seal them: I use red or blue locktight, whatever I have on hand as the budget was really tight for one engine, and tighten the studs into the block so they don't turn. Absolutely the red locktight doesn't leak, I torqued the studs into the block, and when the said cracked mains block finally cumaparts on me I have no doubt the irremovable studs will go to the scrap yard with the block. Moral of the story is use Blue as I found that doesn't leak either... Seriously it's one of the few things I haven't had to do over and that is remove locktighted pre-torqued studs to reseal them. I have rebuilt 1 ARP studded engine due to FOD impacting valves into pistons and finally impacting into the head and piston - it did not blow the head gasket even with a 65 MPH run knocking/pounding like crazy. (Did I mention I have AAA now to not have to listen to krap like this again?) Different slugs and heads and the engine is still running today.
