if your staying within 5000 RPMS Id recomend a smaller carb than a 650, or...a Q-jet, even though a Qjet is actually more CFM than 650, because of its spread bore design, they make greal loe end as well as top end.
If staying with aftermarket square flange for a 350 that never revs past 5000 RPMs Id use a 570 Holley Street Avenger. There are going to be a few in here that call me crazy... but within that kind of RPM thats all that motor needs & it will respond well, infact it would even better with a 4 hole spacer.
Sune... yes an open spacer has the oppisite effect, makes the engine think it has a lil more carb. John