Curious, what sort of power range are you looking at?
Also, yes a 38mm tial will work for a single wastegate as long as it's divided properly.
The biggest downside to the single gate style is the divider up to the gate. When you beat on your car and get that hot, it'll probably get weak and probably break.
I say this because @Furze and I have had no end of issues with the single wastegate on his car. Twice the divider has broken off and jammed the gate open.
If you've also got headwork going into your build I assume you're shooting for more than 350kw. To which I'd say get off the BL/BP headers and get STi headers. You'll run into a wall from 350kw onwards which @SAS will confirm.
So, on that note, bugget allowing, go twin gates and STi headers / up-pipe. It'll make your life easier and if you go to a bigger turbo / cams etc that'll likely and easily outflow the BL/BP headers and single gate setup, you won't need to do it all over again.