Re: 19' trailer 21' boat
I know nothing about single/double axle stuff but post a pic of the trailer so we can see the design.
One problem with sliding up the boat, depending on the trailer design, is that the keel will drag along the center support while launching and retrieving.. Adding rollers may help that.
I moved mine forward about 22 inches and added a break away tongue so I still had room between the trailer and vehicle. It did cause problems launching as I said above, so I had to move it back a foot. I added extensions to the back of the trailer just to have something to strap the transom onto.