This conversion can be done with a slimline kit or a streamline kit.
use the standard 7mm tubes
on the nib tube glue a 27mm long blank flush to one end make sure the blank has been reemed or disk sanded to length beforehand turn blank to desired shape on lathe or fit rubber comfort piece.
drill out a streamline centre band to 7mm
on the other tube glue a 76mm blank flush with one end of the tube turn on lathe to desired shape
insert nib into nib tube then insert mechanism into other end check that you don't insert the mechanism to far.
place centre band over mechanism and up to short finished blank insert refill
push finished long blank over mechanism to meet the centre band simples
hope these instructions help plus pic of pen in parts
Ian