Welcome Rob and nice litst of mods on your car!
It is not uncommon for the MBC to boost creep especially at cold temps...just an FYI, most people install an electronic boost controller to address this issues. I am running the TurboXS HighPerformance MBC and it boost creeps some what but the car is running fine, and I am always very watchful at my boost gauge as I put my foot into it.
A word of recommendation, get rid of the 2.5" exhaust and go 3.0" full madrel too, your car will thank you a million times over AND so will your butt dyno!
I would prefer the SDS over an AIC, yes its more $$ and more of a headache to install and set-up, but SDS has great customer support, there are many folks running this set-up on their Turbo Datsuns and making sickening power. I prefer a good standalone over a bunch of add on boxes like the AIC. Alternatively you can look into the Megasquirt standalone EFI or converting your car to the 1984 and newer 300Zx Turbo MAPHot wire EFI, there is a sticky on this great thread in the Turbo section of this forum. Both the Megasquirt and the 300ZX EFI is much cheaper than the SDS, if you are looking for something a little cheaper.
The Garrett off the Volvo 740 should fit your Datsun with no issues.
I also recommend the boost sensitive Aeromotive FPR (very inexpensive from Summit or Jegs) to control your fuel pressure, quite a few of us are running this FPR with great results.
Hope that helps and answers your questions.
Regards - Yasin