1155 and 1156 are exactly the same. 775 is a tiny bit smaller. Some people have had luck with forcing 775 to work, depending on the design of the mounting. Also, some boards have multiple mounts. For instance my Asus P6T board has 1366 and 775 mounts and my wife's EVGA board has 1156 and 775 mounts. Perhaps there are 1155 boards with 775 mounts?
The best way to future-proof is to save money and spend it on future products. (Ken g6)
SSD turns duds into studs. (JBT)