Hi Sparky77 . . . glad to see you digging in and doing some research. There is alot of resources online, but it takes time sort through and make sense of it.
First things first, Canadian GMC (light trucks) never shipped with GMC engines - they got Chevrolet engines. So the 235's you have encountered are correct for the application. A GMC 9300 in Canada is essentially a US Chevrolet 3100 with GMC branding.
The other thing to keep in mind is that you are on the cusp of a transition period. A '55 1st Series Truck had a block with small cam-journals (introduced in the '53 Corvette complete with solid lifters). Second series '55 Trucks came with the new larger cam journal block casting. And that 3836848 head will fit on either.
The GM Parts Wiki is the place to search:
Fitted Block: Parts & Accesories Catalog November 1962but it can be frustrating - as not all production casting numbers or parts numbers were referenced in print. Keep in mind these are Service Parts catalogs. Many parts were used in initial production but never placed in inventory (and therefore cataloged) as replacement parts.
As for the T56X - that's a 235 built in Tonawanda "T" in '56 for a 3100 truck "X". See attached.
regards,
stock49
