Tank not fitting all the way over rubber and frame backbone?

On the late 70's bikes, how far down should the tank sit on that rubber pad? I push the pad all the way up into the tank tunnel, and then try to press that and the tank over the frame back bone, but it wont sit all the way down, has about a 3/4 - 1 inch gap between bottom of rubber pad and frame backbone. Seems really tight and I don't want to F'up the tank. I was thinking the thickness of the powder coat may be interfering, but wanted to check on how much force it should take to press that down.

Is it the correct rubber u shaped rubber ISO pad? Put in the correct spot? Try silicone on the rubber.

Yes, it is the correct part - non-serrated. I put it a few inches ahead of where the backbone starts to angle down. I saw some other posts where they had to scrape down the leading edge of the rubber a bit. In any event am I correct that it should push all the way down?

Look in the tunnel of the tank. I think I saw indentations for this rubber to slip into. I'm not sue which Years had them. I'd make sure the rubber is in the correct spot, lube it and push it on.

Put the rubber on the frame first, then push the tank over it. You may have to adhere the the rubber with some adhesive or tape around it. That way the tank tends to push the rubber down over the frame, when it is in the tank first, it tends to bunch the rubber.

From memory ... when I built the 390 Auto, the rubber sits about 3" or so from the back of the tank. I can just push the rubber with my fingers when the tank is fitted. I move the rubber back and forth slightly to raise or lower the tank to make sure it clears the exhaust. To ease fitting you can use some red rubber grease. With the exhaust fitted put a fair bit of weight on the tank to simulate it full of gas, and you partly sitting on it ! Ensure the tank seam does not touch the exhaust. This obviously only applies for a cross-over chamber.

The rubber is old and hard, I've had to grind the outer part of the rubber to get the tank to settle down.