I haven't held the Walbro 400 in my hand, but the images I found show a big bell on the bottom where the actual pump mechanism is. So it may fit just fine in a "pump on a stick" installation but it won't drop right in to a fuel module the way the 255 and such will.

I used the stock S4 sending unit.I used a large hose clamp to secure the pump to the "pole" of the sending unit and just used a razor blade to shave the sock grommet to fit. Took maybe 1/2 hour to fit. Probably not as easy as a 255, but the thing flows like crazy and is very quiet. With a quiet exhaust I can't hear the thing run at all.

...did anyone of you who kept the hardlines use E85?

I talked to a non-FD guy (but an absolute E85-Pro), who told me to swap the hard lines if i'm not sure about the material - would be cheaper than an engine failure if the material of the hardline reacts with the biofuel...

E85 requires about 30% more fuel to make the same hp as gasoline. Rotary engines require more fuel than cyl engines to make the same hp.
So if i was told 3/8", -6, is good for 700hp, I wouldn't go past 500 to 600 hp on a rotary. Now add E85 in the mix I wouldn't go past 400.
It's not a matter of if you can still make power on small line, but the reliability and i rather be safe than sorry.
I had a project Datsun 510 with sr20 and people were claiming they were using the stock 1/4" fuel lines for their efi conversion.

Good point, not to say i would hate to install new lines if the smaller one fail with E85, as the price difference isn't significant, i will go at least with -6 - it would be wrong as the upcoming setup def. has the potential to put up numbers above 600...

Found turning up a bit of nylon that slipped around the motor section of the pump, helped in mounting. The outlet barb, I found was 9mm on the minor diameter, so presumably either 5/16 or 3/8 line will work.