I personally took off the cover/fan to plug everything in, it was a pain in the ass to get SAS cables in there.
I just used a SAS 8643 to 4 SATA/SAS break out cable with no side band:
You could easily...
We have a couple of 2950G and 3950Gs in our racks doing the same, we just put a server below it so it doesn't sag.
You do kinda have to lift the switch up to slide it in after servicing the server though.
See my answer in your other thread:
That said is there a reason you're doing truenas on a proxmox rather than just direct install of truenas?
Tons of options, I like option 4 because those sata doms are neat af and you dont have to use a drive slot for the os drive. Or if you already have a sata drive the caddy is easy too, no one needs it with flash drive or idrac iso boot nowadays anyway):
Pretty sure there is a onboard sas...