is it a straight fit for the fitment of a set of RC30 wheels measuring Front 17" X 3.5" and Rear 18" X 5.5" on an Nc30. Or is there something that I would need to do in order to fit theses on my NC30

RC30 front is 3.0" standard, so keep your NC30 3.5" front.
You will need RC rear spindle nut and nut spacer, not sure about clearances and offset, but seen it done [17" kit] on a race bike.
18" tyre choice is limited and 5.5" is wide for a 400? [ 170 tyre]

Dave, so what sized tyre do the RC30's run on a 3" wheel? surely not a 110

Tony,
120/70 crossply is standard, modern radial does not sit well on the rim, a bit too pointed.
NC30 front wheel is best, or kit magnesium 3.5.

to the OP, may i ask whats the appeal in fitting a wheel you cant find a suitable modern tyre for ?

That was my thought, maybe he has just found a cheap pair ?

Dunlop Roadsmart 170 seems the best at present, but need to buy them from the USA, and expensive too.

I am using an old 18" BT54 that is OK, [RC30] but I am now inclined to fit a 17" kit wheel and new Super Corsa pro SC0 from a spare bike. [ride height adjuster fitted already]

So, OP fit 17" not 18".