Alex - Here's a little info (below) I was able to find from a couple of resources on your "donor truck", including a VIN decoding chart (which may not be 100% accurate for a Canadian GMC) and a dealers sales data chart for 1977 Medium Duty Canadian GMCs which is probably the most accurate for the truck between the two.
The 2nd source shows the engine should be a GM (Corporate) 366 Big Block and the 5 speed was one of several available transmission options.
I don't know if the bell housing pattern for your 292 will match the Big Block pattern. Also no experience with if that 5 speed will mount up to your bell housing or not, but I'm doubtful.
Someone with more knowledge/experience with the late Big Truck 70's engines and transmissions will need to chime in to offer some hopefully helpful insight.
Bets of luck in your search.
