64 barracuda runs hot

It is a common misconception that the thermostat will control to a precise temperature. It doesn't. As the heat load increases, the operating point will creep up. Of course, once it is fully open, it has lost control. Do you know that is true, i.e. the temperature won't settle out at a reasonable value? Has it ever boiled over?

Making a shroud is tough. Easier to add a pusher fan to the front. That is all my 64 Valiant slant has (no mechanical fan). You might be able to fit a later 22" radiator (my slant has one). The standoffs still let air flow to the full area, even though your opening is only 19" wide.