So sitting on the ground, level, my truck has about 24" between the mounts.

would a bilstein 14" travel shock work? its compressed is about 20.37"

i have 5" all-pro springs, so they flex pretty well. 83' toy.

They'll work at ride height but flexed out is still questionable your forgetting uptravel/down travel. Flex out the truck with no shocks and measure collapsed and extended lengeths of the mounts and use this chart.

Travel Length Extended length Collapsed Length
10" Stroke 25.93" 15.91"
12" Stroke 29.70" 17.91"
14" Stroke 34.90" 20.75"

Do you have bumpstops? I would get bumpstops setup so you springs just go flat or a little more then flex and measure what shock will actually fit.