Len,
A lot depends more so on axel placement. You can do as Tod said and gain some by moving the winch. However that also increases tongue weight. Leaving the boat in the same position it now is, and extending the trailer rearward, will ligthen the tongue weight slightly due to the increased weight (adding frame material) behind the axle.
It is real hard to engineer a trailer rebuild from photos alone. One has to take into account all the variables when redesigning a project such as this. Glad to offer advice but be warned you ultimatly must do the engineering to make it happen.