Buick IS NOT a flat 8. The Oldsmobile straight 8 was a flathead engine, IMHO. My main concern would be with the location of the shifter, and possibly fabricating a rear crossmember to support the bellhousing, plus clutch linkage. The firewall situation could be resolved by using a steel wheelbarrow tub welded into the cowl area like some of the hotrodders do when making oddball engine swaps.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!