I did this in a customer’s brown 1990 9C1 in 1995.
Same chassis as yours.
He wanted an old school hot rod - but a total sleeper.
He purchased a brand new marine 502 BBC from Mercruiser with 990 rectangular port heads.
I put on an Edelbrock RPM and a Holley 850 HP DP and an ultradyne cam.
I built and installed a good early TH-400 with a gill younger shift kit, 121 pump, deep clutch drums, good clutches and new steels, 34 element sprag, ATI Treemaster converter, Hedman headers for a 2nd gen F-body, the car came back in the shop the next day on the tow hook with an exploded 8.5” police rear.
I put a moser 9” rear axle complete bolt-in unit with 3.50 gears and Willwood dynalite disc brakes.
You will need better front springs for this conversion, I strongly recommend Canuck Motorsports HD springs, with their matching rear springs, and use Bilstein shocks spec’d for an 89 9C1.
It’s a good thing gas is cheap over there, cuz you’ll be using quite a lot!
If you do go forward and do this upgrade, and have any questions, ask and I can help out.
For myself, I have a boxed & cross-braced stock chassis 1988 LS Brougham Caprice with an old school bow tie SBC making north of 450 HP on pump gas with AC, hydraboosted 4-wheel (KORE-3 sourced) late Vette disc brakes, and I truly appreciate how much better the car drives, handles and stops with a SBC, I feel iron block BBC engines are better suited in speedboats, tow trucks, and all out drag cars.
But hey, that’s just me, 450HP on pump gas and the biggest street tires I can fit under the stock sheet metal is pretty good - it’ll light up the rear tires (295ZR50/18, Nitto NT555s) any time I hit the go pedal. Also accelerates hard all the way up to 140+ MPH, just as straight and stable as can be. And, if you like tight handling, use SPC F-Body front upper and lower control arms (with extra positive caster built into the geometry), varishock double adjustable shocks and a good matching pair of HD swaybars.
Good luck & have fun!