Venting of Lycoming 0-290 D Engine

Hello
I have been helping a young fellow EAA member with the restoration of a
EAA Biplane , he is using a rebuilt Lycoming 4cyl. , 125 H.P. 0-290 D engine
in this plane and has a 3/8 " O.D. aluminum tube running from the firewall
and exiting just ahead of the tailwheel , he wants' to keep the underside of
this fabric covered plane as clean as possible , the vent elbow that exits
the top forward area of the crankcase has an I.D. of approx. 5/8 " , he
plans to put a reducer to make the step from 5/8 " I.D. to approx 1/4 " I.D.
on the alum tube , this tube will go from the firewall to the tailwheel ,
the question is this , does the engine vent just relieve pressure and will
the long narrow tube cause any problem ?, am not an expert in the dynamics
of this of this area of the engine , any thoughts or help would be
appreciated .
Thanks
Phil Lohiser
EAA 12873