Congrats on getting the Imperial.
If the radiator is rusty, then the block's water passages are most probably where the rust started. Which can mean the heater core is similar.
Might try a rinse of the system to see what's there. Just a rinse with no cleaning agents. Then inspect the core plugs on the side of the block for seeps. Eventually, though, they will most probably need to be replaced to get at the accumulation in the rear section of the block (which is the lowest point).
The other part in this situation is the heater core. Hopefully it will come out clean, just do NOT reverse flush it! Only flush in the normal direction of coolant flow through it, otherwise, it could clog. BTDT
Good luck,
CBODY67