To answer your question you need to address all of these:
1) You need to decide on what kind of flow you want. As far as how much flow SHOULD you have for a 29g I think the rule of thumb is 10-20x the tank volume so about 300 to 600 gph for your tank. What kind of corals are you keeping? I think SPS like it higher flow, some softies like it a little softer.
2) You need to know what the max flow rate of your overflow is so you don't overwealm it (maybe 600gph if it's drilled at the top or a siphon overflow?) (Assuming you're talking about a sump and not a closed system)
3) Look at the flow rate for the pumps you had in mind at 3.5" head.
4) Reef artists posted the flow rate reduction for SCWDs, apply that to the answer for 3.
I have a 780gph pump going through a SCWD at 3 foot head on a 40 gallon tank and it's not enough flow for my LPS/Softie tank without extra powerheads.