I wish I had something positive to contribute.
I guess if it was me I'd carefully check the block deck and head surface for flatness. Check the fit of a new head gasket and give it another try. A lean condition is not good.
Sounds like you pushed the little engine pretty hard with sustained 70 mph cruise, pulling a load through hills and she got a hernia. I did that lifting a refrigerator by myself.
What rear gear are you running? 2 of the race car tow vehicles at the Clay City race were slant powered so it can be done.
I'm really surprised Big Daddy didn't throw a 392 Hemi into it overnight.
