The '65 was the first year of the 240ci engine in the Econolines. You don't mention whether it's a van or a pickup. If you were to look in the bed of the pickup, you would notice that there is a slight extention that was made of the cab to accomodate the larger engine block that is the 240. The 240 is the little brother of the 300 (same block).

A simple answer to you question is "yes". The 300 will bolt into the Econo. Now the rest is that with the 240, they used the heavier front axle, front springs, brakes etc. The rearend was also updated to a Ford 8" for the greater power. If you want to do the swap to the 300, read up on the 240 Econos.

Ford's inline 6 engines were all very reliable. What you have in the '65 unless someone changed it at some point is the 170ci, not the 200. 200ci engines were dumped in '65 when they went with the 240.