When I did it a while ago I used the old hole as the bottom hole for the new eye. Reason being that the eye sitting a little higher was what I wanted, and I knew it was centered correctly.
If you want to relocate, bring the boat as far forward as possible, then locate it to make sure the eye isn't going to hang up on the V-block, I prefer to locate them so it's above the V by a safe margin.
My old boat would hook the eye over the V block on steep ramps making it hard to launch, and all but impossible to get back in proper position when you retrieved it, so that is one thing I double check when I place them.
Ken.
Edit: make a new backer block, I don't think I've ever seen a boat with block that didn't look too small. Make sure you use plenty of sealer.
Also, I really prefer the stainless U bolt styles, they just plain seem maintain the same 'look' a lot longer than the plated-diecast styles, and are a lot stronger.