My guess is that it is a mid 60's 283. That long filter canister was a big truck item in thole years.
Check the block casting number on the block at the back of the engine on the driver side.
The casting number will be in this location
This is a 350 casting number so it won't be the same on yours. The easy way to check it is to hold your digital camera by the firewall pointing down at that area and take a few photos so you can read the number. Then go to one of the sites that lists casting numbers for small blocks. This is one of the lists that most guys use
http://www.nastyz28.com/sbchevy/sblock.php
If the engine runs good and is a 283 or better yet 327 all isn't lost as those engines are pretty popular with the traditional hot rod guys who are hunting small journal engines to use.