Best fit really doesn't apply.
255/50R17 (27.03") OEM for 94-96 B-sedans with WX3 or WX8
(doubt this fits in the wagon's spare wheel well?)
225/70R15 (27.40") OEM for 91-96 1A2 Special Service Wagon, and 91-96 B-sedans with L99 4.3L V8
215/75R15 (27.69") OEM for 91-93 9C6 B-sedans - Taxis - with LB4 4.3L V6
235/70R15 (27.95") OEM for 91-96 B- & D- sedans with LT1, OEM for 91-96 B-wagons with V92
225/75R15 (28.28") OEM for 91-96 B-wagons with 2.56
235/7515s - 28.87" - were ordered by mistake for Stuart's Taxi in Westbury, Long Island. He was so miserly, he didn't bother correcting the mistake … which, as it turned out, didn't need correcting.
Despite worn shock dampers and saggy springs (said he was cheap), I once put 10 college girls - 3 in back, 4 in between, 3 up front with me - in a 95 wagon going from CW Post to the Nassau Colosseum.
How I drove back then (worth the tip), of course I bottomed it, if only just. I [was ordered to] never put that many people in a wagon again.
Other drivers regularly put 6 or 7 riders in their wagons wearing 235/75R15 (28.87"), and we all of course would put 5 riders plus luggage jammed to capacity from Westbury to JFK or La Guardia (20 mile drives).
The only MILD rubs were when the wagons were fully loaded, and that's only because the owner was too cheap to buy stiffer/taller springs.
I'll grant that GM never spec'd 235/75R15. I'll grant the look was a bit off - not that anyone cared.
Doesn't change the fact that 235/75R15 (28.87") work just fine with Caprice-spec springs.
Too bad 255/70R15 (29.05") won't fit in the wagon's spare wheelwell.
If you're riding 'lower than Caprice', 235/75R15 would be a problem, sure.
From my spotty memory, I remember that going taller than 29.5" required custom-individualized rear control arm geometry, taller springs, and the inclusion of every 'optional' cushion/seat between the springs and their contact points.
With 17" wheels, you can choose any tire 235-wide or wider, until the sidewalls scrape against the wagon's inner wheel wells. I think that the upper limit, without modifying the inner wheel wells, is 275-wide, but some wagons are not as 'consistent' as others.
Between 27" & 29" tall, and between 235mm & 275mm wide contact patches, you have a considerable number of tire sizes to choose from, even if you limit yourself to H-speed-rated tires.
Do NOT settle for less than T-speed-rated tires for our cars, except for winter tires.