Hi bluescope,
An extension tube will only make focus worse as you will need more infocus with your setup. If you are using a standard 2" R&P focuser with 350D and the MPCC, you will need a lower profile focuser to achieve 'infocus'. If you have the newer type MPCC, take the outside ring off . This will give you slightly more infocus and don't use a prime focus adapter. Just use the canon t-thread with the MPCC only. Another method is to move your primary mirror closer to the focuser by tightening the collimation screw.
welcome to IIS btw
