The joys of the Dirst naming system - does ANYONE have a clue what any of the bits are/do?

I've got a couple of 605's I use for MF without any real thought. I take it you've got the colour diffuser head - otherwise you need a new condenser too...

The standard neg holder is the sironeg, which is glass (top and bottom), with adjustable blades for masking - takes any size up to 6x6. 6x7 probably won't fit without a crop, but you might squeeze it in. You don't need the drop in masks that some of the other models use.

I take it you've currently got a siriotub lens board - the recessed version? I think you might get away with it with a longer lens - I'd just try it and see. Worst case you'll not be able to focus at all enlarger head heights. The siropla is the flat board (I think), though I've got two which are slightly different: one is deeper than the other, through they're both badged the same.

The flat and recessed lens boards are surprisingly cheap - about 12 UK pounds. The extended board for VERY long lenses is much more expensive.