[Dead Parrot]

Any idea why it seized? Lack of oil? Got too hot? Loud clanking/grinding before it stopped? You said you tried push starting, so it has a manual transmission? Keep in mind that on a 39 year old truck, almost anything can fail. Wore out brushes on the starter can cause that to do nothing, no clicks or anything. Went through that a couple of weeks ago. If you got it too hot due to a stuck thermostat, you can crack a head gasket and possibly hydraulic lock a cylinder due to coolant leaking into it. You can pull the spark plugs to check for excessive fluids.

If you decide to swap the motors, verify the donor motor is worth the trouble before doing the swap.

A good camera is your friend. Take lots of pictures. Both of the donor truck and yours. Very helpful when trying to figure out where that weird wire or hose goes.

Clean and degrease both motors before starting.

Plan on needing at least $200 in various small parts to complete the job. Just seems to work out that way.

Consider taking the transmission to a shop for inspection and possible seal replacement. If manual transmission, also have the clutch checked out. You will have most of the removal/installation work done anyway as part of the motor swap.