[73 4V XB]

Think about your insurance too. If the car has been modified, bigger engine, different transmission and not noted in the policy they may void any claim. There’s also rego to factor in depending our your state. Is it registered appropriately and so on.
Your best bet is to get a mod plate done to be safe.
As for price, anything with a matching number will fetch more coin. Some strengthen was done to factory V8’s depending on the make and model. I don’t know all of them but take a XA-XC, the radiator support has a double skin across the top while the 6 didn’t. There may be other things to consider, these are just of the top of my head.