Well were getting somewhere with your rig, under powered and over weight, no wonder fuel economy was dreadfull.. I recon with a trip to Jenny Craig, 1.6km/1lt is achievable and maybe 1.7-1.8 with just your wife and kids...
Dont drill any new holes, it turn your transom into swiss cheese...
Cheapest alternative is to have a 25mm spacer plate made, bolt it to your transom and have new engine bolts redrilled into the space plate that have a 75mm higher off-set then your origanil bolts..... Does that make any sence...
This is a photo off my spacer plate I had made to raise my engine an extra 30mm.. Note how the top transom bolt is as high as the transom itself..
You can also see the origanil location off the transom bolts that are now used to hold the spacer plate on... You could call that spacer a jacking plate.....