Good fortune with your install. Sounds cost effective.
The 103 to 123 thou piston short fall in L code (1969-1979) and C-code (1980) US 250 would be fixed by that 6.058" rod and perhaps a decked 22 to 28 cc Aussie 250piston. Great stuff.
The Barra engine can't use the 1.500 to 1.53 deck 200/250 size pistons unless they are custom Mahale/ACL 1.163", or if you won't down in rod size back to a Chev 5.7, or Chrylser Hemi 265 5.75" rod. So the Barra is essentially a new deal, with the little possibility of any option but true Aussie bits, Exception is those 6" Chevy based Manley rods.
The 160 teeth 1965-1992 OHV, 1988-1993 OHC, 1993-date SOHC/DOHC Aussie blocks also cost a lot when it comes to getting easy autos and T5 gearboxes. The nice 2V/X-flow/SOHC /DOHC heads and EFI systems don't make up for the cost of having to integrate either Aussie or US standard gearboxes, distributors, fuel systems, intakes, exhast systems, water pumps, a/c, p/s, alternators, non std non certified emissions issues.
The market for the Aussie 2003- date DOHC Barra and post 1988-2002 SOHC's is in a funny situation because of the ECU's and bellhousing adaptors. The DOHC and SOHC engines are tied in with non US BTR gearboxes and Aussie EEC 4 and EEC 5 sytems without full OBDII capability. In 2008 to 2010, the US market 5R55S was included, but it doesn't have a detachable case, and isn't a SBF or Modular/Romeo/Ohc Cleveland pattern. That's a huge problem as the only easy swap is the T5 1992- 1999 XG, XH Ute with non dual path intake. All the bits can be made to clear the US X-shell (Falcon to 1980 Granada) with ease via a simple Territory sump kit that can be installed, for the S-shell (Fox), its more of an issue but nothing a little Fox 3.3 verses XH sump work, but the electronics won't move forward untill the Explorer/ Mustang Cologne ECU and gearbox can be transplanted. The later EEC 5 Aussie Ford DOHC have sixty sensors which, although they dont have to be hard linked, they do have to be polled or hooked up. They have a restricted handshake with smartlock and other gear, and unless the US guys can use the EEC4 or EEC 5 stuff like the 4.0 sohc can, a potential market will be untapped. C4 SOHC/DOHC bellhousings, where you can fit up a 157 teeth Windsor C9 box or 157 teeth Cleveland C10 allow you to drop the trans controller and go back to the Mustang EEC 4 A9L 5.0 ECU are probably the best bet. There is also the stone age 153 teeth Powerglide 2-stage SOHC/DOHC bellhousing. But parts and back-up have been a bit arrogant. When a US person needs info, they need it on there terms. We Kiwi and Aussies only see things from T5/T45 and BTR 4-stage gearboxes and factory EEC tunes which yield low 15 second quarters. Or turbos which can drop down to 12 or 13 seconds. Then you get ZF 6 speed boxes. We don't appreciate the problems in fitting those to left hooker Mustangs and Fairmonts and Falcons.
The stock US U, T, L, B, C code 170, 200 and 250's are fully supported by a really good Classic Inlines alloy head, brilliant T5 gearbox conversions. The info is there, and the answers aren't 'just buy the Aussie parts and get them imported'. I have lead times of 3 to 14 days on US parts on my 4.0 UR Explorer and 3.3 Fox Mustang, and because they are Yanky, there hasn't ever been a time I coudln't get a part STAT. This is not the case with parts for Aussie sixes.
The Aussie X-flow is a real odd ball, in that is just an amped up 200 narrow block engine, with some of the US 250 parts and a really good Cleveland head. The options are either to dumb it down to a 4-bbl intake, and use a T5 or Toploader or track down the factory 1971 to 1980 C4 Falcon or Cortina auto bellhousing and 157 teeth flexplate. Or amp it up to later EFI X-flow or SOHC/DOHC dual path EFI system. Other advantages is that its an easy roller cam conversion, but cost wise, the transmission restrictions from its 160 teeth four bolt bell that's not the same as the US 200 or six bolt US 250 makes it a costly option.