Re: 7.5 HP Johnson 1982
First things first.
I weigh 250 lbs, I can get a 1956 Johnson 5.5 hp to plane and go nearly 12 mph in my 12' boat. As per my Garmin GPS, So your 7.5 hp should have no issues.
Reason is this, when sitting in the back of the boat all your weight is back there, move some weight forward, fuel tank, anchors, gear, this will help some.
Your balance in the boat is off which is why you have difficulty planing the boat. Whale tails are kewl but not necessary, than can possibly do some damage to your engine, it has happened but is rare.
Another option that most people don't think about is boat planes. They attach to the stern of your boat, near the bottom, to provide lift for the back end. They do work!
They can be added to your boat allowing you to run smaller engines and still get on plane.
Redistibute your weight in the boat, that will help alot.
Regards
Richard