I really looked into doing this exact swap 25 years ago (1987). The Olds diesels were plentiful and the 6.2s were still really expensive. The main issue was the bell housing. Back then the availability of automatics with the dual mount system was limited but the standards could be used with the bellhousing off the 4 4 2 Olds gassers with the 4 speed.
The clutch style flywheel from the 455 would bolt on but I seem to remember there was a balancing issue. Externally balanced????
I agree with Great White in that the 6.x is a better choice, especially if you do any off roading. Just the on again - off again loading of the engine would tend to dynamite the 350 whereas the 6.2 is considerably stouter. I believe the concensus that the 350 should only be used infront of an automatic makes sense. The 350 would be more economical fuel wise but the 6.x is a better choice over all.
Bill
91 Buick Roadmaster/Avant 6.2 NA conversion (gone but not forgotten)
94 Cadillac Fleetwood (sold)
08 Aerolight 23TT
06 Vortec Max Silverado CC SB (sold)
10 Avalanche (electronic quagmire but love the truck)