What year is the block? If it is 58-63, have the person tell you the sizes of the holes in the block for the oil filter lines. If they are 1/2" NPT, you most likely have a 58-63 261 engine.
You can tell 261/235 from block casting code.
In order to know long-distance what you are dealing with, the seller will have to tell you these things over the phone, I guess.
Pete52 has good reason to suggest 58-62 engine - that code looks to be a 235 code (the J at the end).
F1023J : I think on a 261 that the J would be a P (and maybe an additional letter) on a 6xxx series truck (this code might be different for different years and for different option-combinations).
