Appreciate any thoughts on the following: I've replaced the water pump,thermostat, and intake gasket on my '85 351W HO (all oem/ told the intake was leaking). Before reassembly, there was no sludge in the valve covers, valley, white smoke from the exhaust, or oil visible in the anti-freeze- appears coolant entering the combustion cycle. After reassembly and several hours later started to misfire,lost power, then ok to ease home. When home I popped the hood - no pressure in the cooling system and low after removing the rad. cap. While replacing the intake gaskets only applied RTV to the end/side junctions as specified. No leaks otherwise inc. heater core. Thanks.
A miss-fire indicates the coolant is shorting out one of the sparkplugs. I would pull them and see if one of them has a bunch of green crud on it. If it does, then that's where the coolant is going, and it could be a head gasket or a small crack in the head.
You can get a test strip from the parts stores that react to products of combustion in the coolant, which might be easier than a leak-down test. And, some times the leak-down test doesn't show a loss to the cooling system, or vice versa, as sometimes you don't have a leak until you get the engine hot. My 351W had a blown head gasket but it wouldn't leak until I got about about 40 MPH.