Intake manifold blockade?

Technically the thermostat controlled butterfly in the pass side exhost manifold controlled whether exhost gets routed to the drivers side.

Assuming that is functional as soon as the thermostat spring heats up the butterfly opens and exhost no longer is forced to the drivers side.

I'm sure some amount of hot exhost makes it over though.

IMHO I would clean it out as best you can.

If you thermostat butterfly in your exhost manifold functions correctly you have very little to loose.

I live in Northern Colorado, on the coldest days I have no trouble starting my 273. The thermostat spring has not been attached to my butterfly for ever. The butterfly is free to move so I suspect at low exhost pressures, it is partially closed.

I would not remove the heat shield, you don't want loosened rivets falling into your engine. You could drill and tap for bolts where the rivets were and use some locktite.

Cleaning under the heat shield is a challenge, I have heard oven cleaner works well.