Big truck parts comparison

[402Bowtie]

I have a relative fixing some issues on a non-A/C c40 or c50 truck. Have some questions:
1) is the heater core the same on a non-A/C c10-c50 truck? Can we simply order a replacement core for 10-30 series truck?
2a) Are the steering columns completely different or do any parts cross over?
2b) The left turn signal won't "latch" when pressing the lever down. However, by holding the lever down, we can get it to signal. Does anyone know the fix for this issue? We haven't torn into anything yet to investigate.

Thanks!