Unless you drive all the time in first gear on muddy back roads,you'll be disappointed by swapping out the 350 for a 292 6.The 292 will be hard pressed to match the power of the stock 350 V-8 even after installing a cam,headers and more carburation on the 6.
I used to have a small fleet of PU trucks for work,a few were 292 powered,the rest had 350's,all were 4 speeds.The 292's were slower at all speeds,got the same fuel mileage as the 350's and didn't last longer.
There's no replacement for displacement in heavy 4x4 trucks
