YES they are the same.
yes, as long as it has the same box syle body like an 84 silverado would have.
GM never made a 450 tranny, you probably mean a turbo 400 tranny and yes it can be done.
Yes it had 1 from the factory.
simple answer is no.
No.
Can a 84 Chevy caprice classic fit on a 83 Chevy caprice classic
Physically, yes. GM's drivetrain are highly swapable. But there can be lots of differences. I'm assuming you are pulling "x" tranny out of an 84 to put in a 87 that had "x" tranny. Not pulling an "x" tranny out of an 84 to put in an 87 that had a "y" tranny. The devil's in the details. Flywheels might not be the same. Most auto part places have the info and will glad to help.
Yes
yes
No.... You need to change the whole back tail piece
YES.
yes. generally speaking 1984 Chevrolet silverado trucks had a dash in them.