I have the Performance Rocketry 1/4 scale Nike Apache that I've bashed and almost completed to be a two-stage.
I put a 75mm MMT in the booster and the sustainer is 38mm min diameter.
I used the transition to mount the booster's altimeter - using a G-Wiz LC deluxe - not my favorite unit, but I got a good deal on it.
Then I machined an aluminum interstage coupler (a tube) that slides over the forward fiberglass section of the sustainer and caps that. It extends further about 4 inches and has slots to receive the sustainer's fins.
I'm a big fan of delayed airstarts / staging so it was important that the sustainer be able to drag separate and coast for a while before self-ignition....for maximum altitude.
I'm using a Defy Gravity Control for sustainer electronics.
The DG will light the sustainer motor using head-end-ignition (EX Motors) and also handle dual-deploy recovery. I'll prgram the DG to watch velocity and light the sustainer when it's velocity falls below a programmed value of about 300ft/sec.
At some point (can you tell I'm behind in rocket building this season), I plan to add a MAD to inhibit sustainer ignition if the flight is non-vertical....it will then just fly as a pure un-powered dart.
I stretched the sustainer body about 8 inches and added a EBay for the DG.
I never spent much time figuring out how to use the transition as iit comes from Curtis. So I not sure I can help you much there. I THINK the idea is that you hollow out the foward end of the transition to the point where the aft end of the sustainer's 29mm motor slides inside.
I wish I could say mine was ready to fly....I got hung up waiting for the 38mm coupler from Curtis. The booster is finished and I've been tempted to fly it by itself to make sure its stable and recovers fine on its own, but haven't had the chance.
When it's done, I plan to fly it on a 3-grain 75mm L-1500 boost (which is all I can squeeze in) and an 8-grain J-400 in the sustainer - rocsim says it will hit about 25k!.
Hope this helps.
Best of luck with your build.
Let me know if I can be of further assistance.