Slat length for bench with two heavy iron sides

Hi,
What can the span be for an outdoor bench with two heavy architectural sides and an iron support band/brace to go between them for each of the following woods:
- Teak
- Mahogany
- IPE
I’d also be grateful for advice on each of these woods, how thick the staves should be (the iron sides have bolt holes that determine how many staves there can be).
Is there any other wood I should consider using? My lumber yard recommended against cypress and cedar and I don’t want to use pressure treated.
Finally, what type of bolts should I use to secure the staves to the iron sides? Stainless steel or galvanized?
My thanks in advance for your help.
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Thanks for all your comments. I very much appreciate them
What I'm wondering is if hard wood like IPE and Iroko are too hard to work with. I've been told that if I try cutting IPE myself with a table saw I will blunt several blades. In which case I might pay the lumber yard to cut them down for me. On the other hand I don't know how expensive this would be compared to getting my table saw blades resharpened. Does anyone know how hard Iroko is?
I know most iron-sided benches are 4' long. I'm just wondering if I could extend this to 5' or even 6' if I used a really hard wood like IPE or Irokl. Any thoughts?
My thanks again for your comments. I much appreciate them
Our timber yard just cut them the length we wanted approx £5 a 4ft x 23/4 ins x 1/2 to 3/4 ins slat but I you would need pretty thick ones to go longer.
They didn't charge extra for cutting. The bench as a metal strap across the slats but they do give a bit with 2 people on it.