1970 GMC C/M Series 4500-6500 Engines
 

In 1970, when GMC switched the 4500 to 6500 series from the H/J hood to the C/M hood already used by Chevrolet since 1967, in addition to the Chevy 292 straight six and the 350, 366 and 427 V-8s GMC still offered their 379, 401 and 478 V-6s along with the D478 and DH478 Toro-Flow diesel V-6s.

The conventional school bus chassis models (B4500 and B5500) were also part of the hood change.

Is it true that the 1970-72 (and 1973-74) GMC C/M series really had both Chevy and GMC engines available?

~Ben

Re: 1970 GMC C/M Series 4500-6500 Engines
 

I think you are confusing 2 different trucks. The 1966-1969 medium duty GMC 4500-6500 trucks were the E series. The E series used the same cab and hood as the H/J series, though the fenders on certain versions were narrower. In 1970, the E series was replaced with a GMC version of the Chevy C/M series medium duty. The E series was available with the Chevy straight 6, GMC V-6, or Toroflow diesel. The 1970 GMC C/M series was available with Chevy straight 6 and V-8, GMC V-6 and Toroflow, and the Detroit 4-53. The GMC V-6 was not available in the Chevy C/M series before 1970. The 1966-1969 conventional GMC school bus chassis was the S series, and it was basically just a cowl version of the E series.

Yes, from 1970 through 1974 both Chevy and GMC medium trucks were available with Chevy and GMC gas engines. Toroflows as well.