If it were me, and that 235 on offer was still running, I'd drop the 235 in and bolt it up (mod the water pump for space though) while I took the 216 through the upgrade Jerry describes and then put the 216 back in as a 250. Like that your truck will still be useable while the overhaul and upgrade is underway (and likely to take a while).
My '49 3600 had the 216 swapped for the 235, and parts for the 235 have never been a problem, but the 235 is still a tad underpowered imho. Would love a 250 in 216 guise.
