According to this suffix code chart, "CTL" means you could have a 350 or a 400 small block from the early to mid-1970's. A 400 will have a very large harmonic balancer with a thick balance weight cast into part of the outer ring and a weight on part of the flywheel if it's equipped properly.

http://www.nastyz28.com/gm-chevy-codes/chevy-engine-codes-suffix-3.php

It doesn't appear it's an original equipment engine for the vehicle. What year is the truck?
Jerry

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