Is it possible to combine a DT1 (piston port) top end with a stock DT2 (125mm connecting rod) bottom end?  My understanding is that combination requires converting the DT2 bottom end to a 130mm connecting rod?

You're right--can't do it without change.
Now ya know what i'd do so Dont Do That lol. I'd cut 5mm off the top of the cylinder which would give Go Fast exhaust port timing [& transfers sort of] Then other complications arise with inlet port. I've done that on a 360 with some success but need more compression. As i said--don't do it. Oh ok, you could take some off the top & the bottom. Na, go change the rod.

Thanks for confirming!