Glow plugs are easy on the n/a 603 , I have changed all of mine with manifold still fitted.
On cylinders 5&6 you need to use a cable tie to hold the throttle linkage open so you can get hand in there.

Hardest bit is refitting the nut on the glowplug thread and getting it started, my hands are quite large and when reaching in through manifold it can be awkward to get the correct angle with fingers to get it on and eventually I used abit of fuel pipe with the nut pushed in the emd of it and then threaded it through manifold and then twisted pipe to screw nut on.

One more point is that all my glowplugs came out easily.