Just my .02 here. Save the money and get a 148 mag. Do it once and be done with it. I've done a lot of glassing, shaping etc and it's just not worth the grief to me. And your hull is committed to that pump size in the end, which may be a bummer. By the time you figure in your hours and glassing materials, you would have been able to pay for a couple of mag pumps. And no matter how you tune that stock 155 pump, it will never be what the 148 is as far as performance goes.
I'm not trying to be mean at all, just trying to save you time, money and heartache in the long run.
Here's a link on a very well set up 148.
http://www.fastelements.com/p-1243-h20-designs-148mm-torrent-pump-kit.aspx
I'm running a 144 mag with a 14/19 hooker behind my 964. I'm also at 5,000' elevation which takes away some power. But there are a few 964's running 144 mags successfully. But the 148 is preferred. Hope this helps.