Chevrolet (like GMC) got out of the heavy-duty (US Class 8) truck market in the mid 1980s. The current comercial line-up of Chevrolet trucks includes the Kodiak, W-series, & T-series trucks. The biggest Kodiak is the C8500 tandem with a GVWR of up to 63000lbs, and the offer either the Isuzu 6H diesel or the Cat C7 deisel.
The latest (and only) brochure I could find for the Bison was 1980. It didn't mention anything about BBC dimensions, but I did get a good list of available engines. The standard was the 8V71N; or you could choose one of the following options depending on the model you wanted (singel vs. tandem rear axel): 6-71N, 6-71T, 6V-92TA, TTA, TTAC; 8V-71TA, TTA TAC; 8V-92TA, TTA, TTAC; NTC290, Formula 290, NTC 350, Formula 350, NTC 400, or 3406.
Right out of the box your Bison kit represents a 1977-78 truck.
Hope this helped.