Leaky 400 RB... Any suggestions???

Worst case you warped a head/ the block

I would CERTAINLY try your approach first. I assume at this point you have not tried to just retorque the head bolts? If the bolt threads are leaking, they would probably be seeping into the oil.

So at the very least I'd do a careful retorque, and I 'spose resealing them is not a bad idea.