If you're going to use angle iron, put it under the cab floor and drill and bolt Grade 8 3/8" or 7/16" fine thread bolts and big flat washers (3" or so in diameter and 1/4" thick) to pull the angle up against the floor. A bolt around 3" long or so can also be used to attach the seat belt bracket and it will spread the load better than an above-floor angle iron.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!