in relation to the body structure, "radiator" is the assembly, not the fins n tubes - the core support is the front of the triangle that mounts and aligns the entire front .... tho that doesn't explain your bumper going limp, which could have a variety of unrelated causes, including something loose enough it 'just moved' .... did you take out the grill and remove the hood for the engine swap? .... have you inspected all the rubber involved, like under the core support and cab? considered weight change effect on the old front springs?

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics