Is this a new rebuild? If so, you'll get a knock if the rods were installed with the pins towards the rear instead of the front. If old, no you can't get the heads off without taking the exhaust nuts off and at least pulling the headers out of heads. No you can't get to the rods from the pan..maybe on a pre-69 engine where the cam was still on top but I don't think so. If the oil pressure stays constant at around 80psi above idle, you've got pressure..if low, suspect the oil pump gears or a clogged oil gallery. Might as well pull the heads and get to the rods. Bet you can feel some give when you pull on the rod...only takes a little to knock. Pull the rods, measure, balance, assemble. Chris, if you're in a hurry, I have 3 rebuilt short blocks on the shelf (new chain, gear, seals, clutch pack, etc, sitting on the shelf for exactly this situation. Just swap out the short block.