Providing nobody comes up with any show-stopper comments regarding the latest (Harken) components we've selected, I'll order them this weekend and get 'em on their way.
I may be late to this party, but I thought that I should mention something that I didn't see spelled out above.
In the world of mechanical advantage (ropes and sheaves, levers, gears, etc.), you don't get anything for free. I saw it mentioned many times that your sheet loads are too high, and that it may be due to the 3/4 boom attachment, which is certainly possible. It was also mentioned that you didn't want too much line piled up in the cockpit when close-hauled. But remember that by going to more parts of line, the line-pull is lower while the amount of line needed is increased, but the change in distance between the blocks is reduced, so you can end up in the same place.
If the two blocks were in the same horizontal plane, then for a given sheet pull force at your hand (ignoring friction), and the same horizontal angular change in the boom, a 2-part system at the end of the boom is equivalent to a 4-part system at mid-boom. Same force at your hand, same amount of line used to bring in or let out the boom a certain angle. You'd be bringing in and letting out the same amount of line during a jibe.
So, assuming the original main sheet system was 4 parts at the end of the boom, and you now are connected at the 3/4-point, you would have 4/3 the force at the boom connection (for a given wind load and boom angle), resulting in a 33% increase in the force in your hand (not counting friction, which is substantial with low quality blocks). You would also only use 3/4 of the amount of line it took with the blocks at the end. You pull less line, but you pull harder.
To bring the single-part force and amount of line used back to where it was with the original configuration (assuming it was 4 parts at the end), you'd need 5.333 parts of line (4/3 x 4 parts). That leaves you with either 5 or 6 parts. 5 parts and you pull less line, but pull harder; 6 parts for a lighter hand, but a bit more line gets used.
If the boom attachment was at the 2/3 point, then a 6-part system would give you exactly the same force at the hand and amount of line used for a given angle change as the original 4-parts system at the end of the boom.
The whole problem gets more involved if the fixed block is now at a different elevation than it originally was, making the forces required when close-hauled higher if the block is now lower than it was originally, assuming you're trying to bring the boom to the same angle from centerline as before. If the fixed attachment is lower than original, then 6 parts is more likely to emulate the original configuration, though with a cost in amount of line used further off the wind.
Mind you, this is just a comparison of the original arrangement to the new arrangement - no commentary on whether what you think it originally had is right or even desireable. And, as mentioned before, the forces in the boom change as you move the block termination towards the mast, as well as the attachment forces going up. Even the reaction at the mast connection is different.
But I just thought that I should point out that if you match the number of parts to the relative boom location, you'll get the same force at your hand, and the same amount of line used for a given angle change, no matter where you attach it. Force times distance for a given boom angle change can be the same regardless of how many parts are used if you terminate them at the right spots and use low friction hardware.