It would be a 292 if its in that late of truck. It could be any size y-block though. It it has random numbers cast by the dist and numbers above the generator its a 272, if it has casting number abouve the oil filter its a 292 or 312. A 312 would most likely have ECZ as the first digits. A 292 would have EDB, B9AE, or C2AE above the filter.Check John Mummert's "FordY" website it has everything in a chart.
A 292 block is what you want to start with. That with a 312 crank makes a good start for a performance Y-Block.