I have a 605 with a semi-flex B from my 400n.
A 800 is a Large pen so sort of clunky and is a nail, unless you can get a W. Germany or pre'98 one.
A 600 and there are so many beautiful ones, is a light and nimble medium-large pen with a thicker girth than a medium-large 400nn.
My advice is to find the 600 that appeals most to you, and buy both a semi-flex to start with and after a few months to get use to semi-flex buy a 'flexi'/maxi-semi-flex from Penboard de.
Sell the blobby semi-nail nib the modern 600's come with. Or send it off to be made stub or CI. There is not much else one can do.
You are just about stuck with the 800's nib in the 400's nibs don't fit.
The 'semi-vintage' 600's are the same size as a 400. Just the deluxe version, the nib is well liked, even if it's not vintage semi-flex or 'maxi'.
If you post your 600/400nn or 400 it is longer than an un-posted 800 and better balanced.
The vintage pens had to have had superb balance to keep market share.
Modern Large pens don't have the balance....and many later generation folks, don't know better.
Gee, now that I got that 400n's nib in my 600....I can go looking for the beautiful ones .....as soon as my money bush gets back from it's winter long holiday in St. Moritz.
Of course I am a bit prejudiced against big clunky Large pens. Do have both the more comfortable to me, '50-60's medium-large and a '70's-80's Large 146. I always feel it takes me time to adjust to my 146.
Got a Townsend, Lamy Persona and a big, light but still clunky Safari. Some long metal Pelikans also.
I could talk my self into a W. Germany 800, got a real, real nice true regular flex nib with a nice amount of spring. It would be worth getting use to.
Too bad you like fat pens....I was shocked when I found out the thin and elegant Snorkel of mine was a Large pen.
Edited by Bo Bo Olson, 19 March 2015 - 15:22.