Who ever told you 350 modified probably meant a 351M. It's a common Ford engine used in mid/full sized passenger cars and light trucks from the mid 70's up through the early 80's if I remember correctly, sharing the basic characteristics of the 400M that Ford started using in 1972. I haven't done much with any 351M/400's but I believe they have the same bellhousing bolt pattern as the 429/460.

As far as what the truck would have came with originally back in 1969, the three common V-8's would have been a 302 windsor small block, a 360 FE big block, or a 390 FE big block.