John, Like Bradford said ....Only use a one piece length of tubing in the original size... 3x3 or 3x4 or whatever the original tube size was. Yes, it's possible to lengthen it by welding, but you'll destroy the galvanized coating at the weld point and invite rusting at what is arguably the most critical point on your trailer. (Don't forget it will rust from the inside out, so you won't see the rust until the tongue buckles and snaps off).
And, Like Phatdaddy said, you might be able to just slide the winchpost forward a bit. Just be aware that you'll change the loading on your tongue as you move the boat closer to the front of the trailer. You may have to adjust the wheel carriage to compensate. Remember (as a rule of tunmb) you want 10% of your boats weight on the trailer tongue for stable towing.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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