I had the exact same symptoms on my '79 Capri, dude. Turns out it was the intake manifold gasket leaking in the left bank rear corner. Frickin' RTV!!!
That crap creeped up behind the gasket and made a virtual river of coolant spew into the lifter valley. Talk about being pissed off...
Anyhoo, the things I learned from that:
1. RTV is
EVIL in a TUBE. Use only the required amount.
2. The lower intake bolts do NOT need that much torque, no matter how much it feels like it needs more. 20 lb-ft is the maximum.