You could get an idea by measuring from the back of cab, to axle center. Even with the shorter bed, you may still have the same distance, due to axle placement/hitch mount placement.
Seems like GM SBs always had a little more cab clearance than Ford, not sure on the Ram.
I've not needed a slider, with my last two trucks (both SB). The FW front design makes a big difference, most newer ones are rounded. My pin center is back 41"s from the cab right now, with good clearance. I could get as close as 38"s with this FW, and still get by with no slider.
I don't know what trailer you have, some older ones were really square on the front. If a slider is needed, I would get a good automatic one, and not worry about it.
Hope this helps you out,
Jerry