What transmissions are we talking about here? If you have a TH200 and the '84 has a TH700 (which it should), you'll need another crossmember, shifter and driveshaft.
Exhaust manifolds/headers will be a challenge, since the Buick parts on the engine now (what model is this car?) won't fit your F-body chassis, and the rest of the exhaust will have to be custom fabricated. The engine harness probably won't be a direct fit, either, but that shouldn't be too much of a challenge.
I had a '77 3.8 V6 and 5-speed in a '76 Vega. It ran a best of 17.10 at Scribner with headers but completely stock otherwise. The '84 engine probably has more than my 105 HP, though.
Do you consider CC carb to be "old school"?
As to whether it's a good idea, it will probably be as much work or more than a V8 swap, and the car will probably be worth less than it would be now with a good engine. You'll have to weigh the pros/cons of a "free" engine/transmission against the hassles you're sure to face and the lack of resale value. If you do it, I'm sure you'll find in the end that it was not actually "free". But, you'll be a better person for the experience.
82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4, 2300 stall TC. Ported World 305 heads, Crane PowerMax 2050 cam. ZZ3/4 intake, oil pump, pan & baffle. Accel HEI SuperCoil & module. Hooker 2055 headers, 3" Catco cat & 3" catback w/dual-opposite Flowmaster 80. 2.93 limited slip. Spohn SFCs waiting to be installed. AMSOIL syn lubes bumper-to-bumper. Daily year-round driver. Best ET, speed TBD...
57 Bel Air, my 1st car. '66 396, 9.7:1 forged TRWs, Weiand Action+, Holley 750VS w/4150 conversion, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 700 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).
[This message has been edited by five7kid (edited December 02, 2001).]