Modifiying fiberglass dash?

This is my first post and I'm fairly new to fiberglass although, i've read through quite a few posts and tutorials to get a clue before I ask for help. I tried some of the hot rod forums with little luck. I think the knowledge base here is better for what I'm trying to do.  I purchased a universal fiberglass dash for the street rod I'm building because I didn't want to try building one from scratch. 

The ends and top of the dash will have to have material removed and added to fit correctly.  What would be the best method and material to add to the current dash. I have room on the backside for mat but what about the front where the new piece butts to the old?  How do you blend that area so it doesn't crack and is strong and  also flat?  Would it be better to remove the ends and make new ones from molds or add to what's there?

Also, I'm not sure of the best mounting method.  How many mounting points and thier locations would be acceptable, I live in the phoenix area and was wondering how much fiberglass would expand and contract  with temperature or, is this a non issue? 

Any input is greatly appreciated. Thanks in advance.