A lot of confusion and bit of misinformation here concerning mid 60's-early 60's GMC truck engines.
First GMC *never* used Olds or Buick engines in their trucks. The Buick rumour probably started because someone saw a listing in a parts catalog or somewhere concerning a 401 "GMC" engine and assumed that it since it was a"401, that it must be a similar displacement Buick 401 V8. Not so. A 401 GMC engine was just that - a "401 GMC" - and a V6 to boot!
Here's the real deal on the somewhat confusing GMC engine line from '55 through the mid 60's.
U.S built GMC's did use a version of the Pontiac V8 from '55 through '59. In 55 they used a 287 Pontiac, in '56- a 316, and '57- a 347. In '58 and '59 they used a 336 V8 that was a small bore (3 7/8) vesion of the Pontiac 370 V8. To muddy the waters a bit more, the GMC advertising guys gave the Pontiac engines one extra cube in the brochures in '55 and and '56. Hence the the "Pontiac" 287 was advertised as the "GMC 288" and the Pontiac 316 became the "GMC 317" engine. In these years too, if a 6 cylinder engine was used, the base engine was the 235 inline Chevy. A 261 6 banger that was a stroked Chevy 235 was also offered. In the U.S., GMC dropped the Pontiac engines after '59 and came out will their own, all new 60 degree V6 engine for 1960 - starting with a 305 V6. This engine was expanded over the years and depending on the model and gvw rating, became available in 305, 351, 401 and 478 inch sizes-all of them V6's! They even produced a massive 702 "twin six" V12 version for use in the GMC 'big trucks'.
Canadian built 6 cylinder '55 through '61 GMC's used the 235 and 261 versions of the Chevy 6. '55-59 Canadian built V8 GMC's never used the Pontiac engines. Instead, they came with 265 small block Chev V8's in '55, 56 and '57 (and into early '58) with the 283 Chev V8 becoming optional in '57 and the standard V8 in mid '58 through '67. In '62 the 327 and (1962-'65) 348-409 Chevy engines became extra cost options in various model Canadian GMC's. Beginning in '66, 396 and 366-427 Chevy big blocks were used as optional engines after the phase out of the 348-409 Chevies. Canadian built light duty trucks (under 10,000 GVW) never used the big U.S built V6's- they were simply rebadged Chevies and used the standard Chevy inline 6 or 283-327 small block (or 348-409 or 366-427 V8's in the heavy trucks), although the 401, 478 V6 and 702 V12 could be "special ordered "in the big (over 10,000 lb GVW) GMC trucks in Canada.
Hope this helps clear up some mysteries.