Don't know if you'll need half of what you're talking about.  To start,
you might be able to use the same mounts for the motor (recommend spending
the few bucks to pick up new ones since it's out).  I wouldn't change
springs or radiator until it's in the car.  I doubt you'll need 'em.  I
always use all new hoses and belts during an engine swap (since it's out).
 I'm not familiar with a GM200 tranny.  A TH350 should fit just fine in the
'81 Firebird, though.  You might have to get a different tranny mount, not
sure.  Just ask the old & experienced guy at your local O'Reilly's and he'll
hook you up.  If they can't, don't go back...
 As for wiring and a computer, if you're gonna use fuel injection or
something that needs a computer, we need to know more.  Heck, you could keep
it bare bones and use a carburetor and HEI distributor; Voila'!  No Wiring:)
Let me know.
 Best Regards,
 Drink

    The 265 is a Small Block Chevy. so remove old engine, bolt in new 350,
drive away. Pretty much the same with the TH350 as long as you use a
lock up torque converter.

If you want better handling get 81 Z-28 springs & sway bars. They are
not required. the 265 is with in 50 pounds of a 350.
Charles