hi lost boy , lovely looking work , i have a 400/4 turbo , built in '80,
hope you don't mind me picking at something , if you angle the feed to the plenum , you will get no3 running lean .you could have the stub pointing into the middle of the plenum ,at a right angle to it , or do what i did ,which was to mount the turbo with the carb on the lhs of the bike ...so the outlet pipe is just above the gearbox,then run it forward to a right angle ,then vertically into the middle of the plenum . it mixes the air /fuel well at the right angle ....,and feed the plenum with as little velocity bias to any one cylinder as possible .
regards
robert.