Calyx,

 The engine bay of your "H" body Bonneville is the same size as the
"C" & "K" body cars which are both available with V-8 engines
(Cadillac DeVille and Seville), so physically the transplant would not
be that hard given the proper tools.

 HOWEVER, that having been said, just dropping in an engine is only
the tip of that iceberg. You'll also have to change the transaxle (the
4T60/65E will not handle the torque of a Northstar V-8), the drive
axles, possibly the engine cradle and for sure the entire ECM
electronics. Interfacing the Northstar's electronics to those of the
base vehicle will not be an easy task.  Now if you have the skills to
do all that you'll end up with one really nice car, but we're talking
100s of hours of work. You'd be better off trading the Bonne in on a
Seville STS.

 If you still want to go ahead with that swap, I'd recommend you find
a Northstar of model year 2000 or later - it is MUCH improved over the
earlier Northstars and, as a bonus, will run on 87 octane gasoline.

 Think this over VERY carefully before proceeding - you've got a lot
of work (and cost) ahead of you (and legal issues as far as smog law
compliance in some states like California).

Regards,
 Bill Bowen
 Sacramento, CA

P.S. A better alternative would be using a 3800 Series II supercharged
engine (GM VIN Code "1"). The Bonneville SSEi used that engine, so all
the parts are available for a "clean" transplant. With some minor
tweaks the Series II supercharged engine can produce in excess of 300
HP (the HP rating of the Northstar in the Seville STS).