[MHml] Advice please

[Allied]

I have a 40 foot glass over ply, bridgedeck, catamaran and I am about to 
start work on extending my hulls by about 600 mm, changing the transom's to 
scoops and deepening the draft aft, to make up for diesel engine, fridge, 
freezer and various other creature comforts, which have sunk me by the 
stern.

My thought is, to make new, extension keels, from 40 mm wide, laminated, 
timber, glued and screwed to the existing hulls aft, giving the extra length 
and depth. Then filling in the depth and some extra width, with foam and 
then glassing over. Cutting out the transoms, filling between the frames and 
generally faring with foam and again glassing over. Fitting light weight 
steps (yet to be designed).

I have never worked with foam and would appreciate a 'beginners guide', in 
the form of advice, pointing to a web site, or printed material. I also want 
to make a light weight flat deck, for behind my afterbeam and between the 
hulls. Do I use foam glassed over, how can I make it strong yet light (build 
in ribs)? I will probably make the boarding steps as open style, with a flat 
board, how best do I make these flat steps?

I am on the North shore, in Auckland, NZ; where is the best place to buy 
small quantities, of the materials I will require?

All assistance will be appreciated, thanks, Colin