2 questions: Filling in Transom and Extending Hull length

[willy]

Welcome Troller to the dark side!
When you say extend the transom are talking about extending with fiberglass the actual transom or closing in the existing transom and adding a bracket such as a Armstrong or Stainless Marine type?
The extended actual transom was done by the factory in the V21 (1990's)
The bracket on an enclosed transom has been done dozens of times here by guys.
The most common is the boats that were originally OMC Sea Drives as they had an enclosed transom with a small hole cut into it for the controls to go thru. These and what were originally I/O s like your previous boat are the ones that get the bracket added the most.
A boat with a standard notched transom could also be closed and a bracket added but you would have to be an good fiberglass guy to do it right. If your transom is currently in very good condition closing it in would not be that difficult. If it is old and tired or wet then it is much more work removing old and installing new and you could make it anything you want when you do it.
Others with glass experience will chime in but adding the bracket and enclosing the transom has very good results with the V hull. Better performance, ride and being enclosed makes it a little safer in my opinion.
Good luck which ever way you go