It functions but you have to use a case and...
I did this a about a week ago. Here's how it went.
I bought a SPH-L710 without realizing it did not have a sim card slot. The screen had one slight crack and everything else worked.
After receiving it, I bought a T999 with a cracked screen/ non-working digitizer and everything else worked (the seller heard sounds when it powered on.)
I disassembled both and put the T999 mainboard on the L710 screen digitizer. Most of the screws line up. The two screws that don't line up are:
1. at the bottom near the TOP of the USB port. The hole in the board and the hole in the digitizer are on opposite sides of the USB port. This cannot be remedied, where the screw goes through the board, there's no hole on the digitizer, and you can't put a hole through the mainboard to meet up with the one on the digitizer. You could try but I think there's a circuit on the board there, or not enough room.
The two holes on the SIDES of the USB port will line up only when you squeeze thee mainboard and digitizer together while installing the screws.
2. on the side near the power button, there is a screw that goes there, but on the T999 mainboard near where the screw goes, there's something that looks like a button cell battery about 3mm wide and 1.5-2mm tall. It stuck in a hole made for it in the T999 digitizer, but the L710 digitizer does not have a recess in it to fit this button cell so the T999 mainboard sticks up slightly, enough that if you put the screw in the hole in the side, then the power switch on the mainboard will not stick down far enough, and when you push the power button on the outside of the case, it won't engage the power switch on the mainboard. To remedy this, leave out the screw on the side of the mainboard.
Because the mainboard is slightly bent due to the button cell battery, you need to have a case that compresses the phone together so the power button will engage the power switch properly. I used a "waterproof" chinese knockoff case I got on e$ay for $13. It's not waterproof, but the phone didn't get ruined even thought I tested the waterproof-ness of the case before I put the phone in and it passed, so maybe a genuine otterbox case would have been better. But the case does OK to compress everything enough that the power button and power switch work all the time. With the case on, you can't tell the phone is missing screws internally.
Verdict: everything works properly except the camera doesn't focus properly, it only works in Macro focus, I don't know if I should take the camera module from the L710 and try it or if it won't move because of the stress of the mainboard being slightly bent all time due to the button cell battery. It did focus properly for the few few days I had it.
One other issue: I can't get Windows 7 to recognize it, so I can't use Odin to root it, I've tried all the drivers, I've tried all the modes. I have no idea if this is a Windows issue or if it is somehow caused by the Frankenstein's Monster-ness of the hardware.
So yes, it can be done, with a "rugged" style case and with a few issues. I have no experience with the I535 at all.
I hope this helps,