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Be careful with aftermarket radiators (is there anything else these days). On the one I bought for my 87 DL, I found the inner filler neck sealing surface was not at the correct depth for the caps we use. It was too shallow but the radiator cap fit and I didn't even notice the issue at first. The over-pressure relief spring was being compressed too tight so no fluid could expand into the overflow tank. This was not a problem while there was still some excess air in the system from the refill but once the air was burped the trouble started. Coolant would properly be sucked from the overflow bottle when the car cooled off at night, but no fluid could expand out when I drove. This worked for almost a week with no overheating or leaks. Eventually, all of the air is burped from the system which is a good thing up until the antifreeze needs to expand ever so slightly but has no place to go. I started seeing antifreeze leaks in several places, all while the engine was running well within normal temps. To prove I was right that the shallow filler neck depth was the issue, I installed a pressure gauge on a section of the heater hose and before the engine was anywhere near operating temps I was already seeing pressures well above 25 PSI. Sadly, I did not catch this soon enough and it cracked the plastic heater core case and also swelled up several hoses and caused the new radiator to leak at the tanks seals. Moral of the story is to be sure you have the correct radiator cap and test it. I bought at least 5 caps and also tried every cap they had at both NAPA and Autozone. Not one of them fit correctly. I have a lathe and ended up making my own adapter. I could not send the radiator back because I picked it up long ago but did not install it until much later. This repair was done in April and now that we are hitting 110F here in Phoenix and I'm running the A/C full time, I have no leaks. The flow in and out of the overflow bottle is as expected and I rarely see anything close to 13 PSI on that temporary pressure gauge. Highest coolant temps I've hit are 205-208F. No issues at all now.

Thanks for the excellent feedback. I believe I have now been officially talked out of having this engine rebuilt. This is why I come here.

Definite head gasket. Exhaust gasses in cooling system and it's gotten bad enough to show some gnarly bubbles at idle. I really didn't see notice this when I bled the system the last few times. Not sure how long it's been an issue. I am planning to part ways with this beautiful car. Big respect to the 2003 LLBean. What an amazing daily. Was very much looking forward to the drive across the country in this comfy beast. I learned a lot from the car and definitely will take more preventive measures on all the future vehicles I own.

JDM engines are a way better deal and it's a huge risk to do HG's on anything with over 200k. Besides that we just don't do HG's on H6's at all - the cost is way too high as they are much too time consuming. It would cost more than the car is worth and you'll have 200k+ mile parts with new gaskets - absolutely terrible idea. JDM motor plus install would likely be around $4500 through my shop. They come with a 12 month, 12k mile warranty. GD

the level in the o'flow should only go up a few ounces, then those few ounces should be pulled back into the rad during cooling off. kinda seems like you are experiencing a lot more coolant going into the o'flow on a single drive, that does point to headgaskets (more so if you can spot bubbles when hot and idling) if, it takes several drives to fill up the o'flow, that seems more like an issue with the rad cap or maybe some air leak somewhere.

that would be my guess... and they dont typically mix oil & coolant... at least not in the beginning.