The frame can be easily checked using the horizontal and vertical dimensions and methods in section 2 of the factory service manual. The Chucks trucks link IIRC is for the later 73 up trucks and not applicable.
The important thing to do is measure accurately against a flat surface which most garage floors are not. A plumb bob and a correctly used quality measuring tape will get you very close repeatable results. In any event a little time will tell you quickly if it needs "professional help" or is go to go.
Welcome to the board and be advised we LIKE pictures! :-)
* VIN/Model Decoders
* Power Team Charts (engine/transmission/gear ratio)
* Post Your Pickup SPID Blazer SPID Suburban/Panel SPID
* RTFM ... Read The Factory Manuals... download 'em here