It’s not easy at all, access to put the structure under the deck is very awkward without cutting holes in the side. When built this is all put in place before the deck was fitted. The wooden ones are much easier.

My recommendation would be to buy a boat with through deck sheeting if you want a plastic boat! Sorry

I would agree with Chris – it is virtually impossible to retro fit through deck sheeting to an early Speed GP, fitting through deck sheeting to a wooden hull either Series 1 or 2 is possible and much easier.  If buying a Speed boat with through deck sheeting make sure the plywood under the deck is in good condition as it can rot and is very difficult to replace. See my article in Mainsail 2 years ago about plywood rot in Speed boats.