I've had a couple trucks with straight sixes, one 250 and one 292. They are awesome engines and will run forever. The 292 one's body completly rotted off...so it went to scrap. I beat the living p!ss outta the 250, took it all in stride. I sold it to one of my friends unlces...he's still using it to haul wood up north.
However, I wouldn't swap a 350 for a 292. Unless you absolutly needed to get the truck on the road, and it was all you had at 9pm on a sunday night.