good looking truck. the engine is an fe for sure. course that could mean 15 different engines combinations....
360/390 most likely, put #1 to TDC. Measure #1 and #4 cylinder depth with a dowel, mark at valve cover rail. Measure with calipers.
3.5 is 360 3.78 is 390, 3.9 is 410/428
course these days a 360 block with a $1800 rotating assembly makes a 445ci... ohhh yeah.