I had a friend make spacers using 20mm aluminium round bar stock that extended the length of the DL standoffs. However I have seen a lot of different ways to extend your spacers in previous posts in this thread.

One thing that you have to take into account when deciding on the length of your spacers is where you want the CofG to be. I wanted it in the forward position and I still battled to get it there with the 3 3/4" standoffs. I decided to set my CofG in this position for two reasons, firstly this is my first gasser and large scale plane and I did not want any surprises, and secondly I am going to use this plane for IMAC not 3D (I am entering my first comp at the start of February in Basic)

Cheers
Pupmeister