I think you have two choices WRT your blocks: add a cam cleat to what you have or buy a new 2.25" triple with a cam cleat and convert to a 5:1 system. If you go the add the cam cleat route, take your blocks to a marine supply store to make sure everything will work with what you have got.

The OEM main sheet is 3/8" dacron, 36 feet long. 5/16" line is actually a downsize but the higher performance core (Spectra/Dyneema) lets you do this. Smaller diameter line runs freer through the blocks (sheeting out). The bottom ratchet block does not "grip" the sheet per se but aids sheeting in heavier wind.

Sounds like the previous owner may have jury rigged some of the rigging. It may take some upgrading to sort it out.