Sorry I didn't respond sooner. I don't frequent this site too often. I highly recommend the big block transformation.
Getting a good tire/wheel stance on the Seville is tricky. The front track is 2" wider than the rear. As you know the stock front tires almost stick out from the body while the same size on the rear sits inboard way too far. I had the 275/60 tires on the rear only. I was running 235/70/15s on the front but they did rub slightly on the rear of the front fender liner during full lock turns. I then put 225/70/15s on the front and they fit perfectly. The 275 fit fine on the rear but looked a bit too bloated on my 15x7" wheels, particularly compared to the 225s on the front. The 275s might look better on 15x8" rims, but I do not think they would fit the car, since a 15x8" wheel would have to stick furthur out (negative wheel offset) to clear the parking brake hardware. I instead replaced the 275/60s with 255/60/15s on the rear and I am now very satisfied.
My recommended combinations:
Front: 225/70/15 tires on 15x7" wheels with +3/8" offset (my '84 Buick wagon wheels)
Rear:255/60/15 tires on 15x7" wheels with +3/8" offset (my '84 Buick wagon wheels)
OR (for slightly better tire to wheel relationships)
Front: 225/70/15 tires on 15x7" wheels with +10mm offset (custom aftermarket wheels)
Rear:255/60/15 tires on 15x8" wheels with -5mm offset (custom aftermarket wheels)
I also upgraded my shocks to KYB Gas-A-Justs (no more ride levelling).
I used 1970 Olds 442 W and Z code exhaust manifolds. The passenger's side manifold must be installed AFTER the engine is in. It looks impossible, but CAN be snaked in. Once in, they clear everything perfectly. I had a local exhaust shop fabricate dual pipes. My installer insisted that 2 1/4" pipes were the biggeat he could bend over the rear axle. I think 2 1/2" pipes would work and would be better suited for a built 455. I have inexpensive V Force chambered mufflers (Flowmaster impersonators). They sound and work fine but create a load drone at about 45MPH with my 3.08 rear axle. I would like to try something else. The dual exhaust requires removing the stock high pressure fuel pump from the underbody (no big deal).