How do i build a tough 250?



I want to build an engine up over the year for my XF so i can turbo it around the christmas holidays. I will be using a 250 crossflow alloy head. And Straight LPG. I need advice on how to do it.
What parts do i use?
What compression i need and what type of pistons needed etc etc.
Also advice on how to set up the LPG setup with the turbo would be useful too as i have a fair idea but too much knowledge is never enough.
Should i use a crank from an EF as im told this will fit? Do i need to have it strengthened? How?
My head has already been worked and is ported heaps, double valves springs etc. I know ill need a custom cam to suit.
I plan to use the XF EFI manifold as the plenum but that is really as far as i have got.
Please help me I need an engine that will last tht can handle a lot of power.
I dont want to use an OHC engine either due to hassles.
I also want to convert to straight gas in the next few weeks. Will this cause any problems? Will my head be okay as is?

Thanks Guys.
heres some ideas,
the EFI manifold is good, use a GRA gas mixer bolted to it, in a blow through setup, run the EFI or something like a Crow turbo cam.
Use later EF?L rods with custom pistons to drop CR to 8:1, run a TO4 with integral wastegate, get a Mike Vine exhaust manifold or fab a something suitable.
The later EF/L crank is Ok but the crossflow is just as good, the later crank with more balance weights will be smoother, but no stonger/
use standard oil pump. roller timing chain, windage tray. increase oil capacity to around 6 litres, run a cooler and remote filter.
Run no more than 1bar of boost and you should have a terror.
Why not pick an OHC they have the good stuff already.
contact C.O.M.E Racing they can help you in building a tough engine
Thanks for the advice guys.
I might get in contact with COME they do good work.
My current engine has a st 3 cylinder head and cam from them. This was around 1500 and they quoted me another 2000 for a block with all the special stuff to match.
Ill ring them when i have a bit more cash and a donor engine ready.

i was going to get there stage 3 x-flow motor but i ended up getting the same engine built but locally
How much did it set you back and where did you get it done? COME is about 45 minutes from my house.
How does your xflow go now? Mine is awesome with the head, cam and new manifold and 4bbl carb. Revs to around 5500 now easy not that im game to so it very often.

mine set me back $2440 plus the 1000 odd dollars of new parts i put on it it goes well but is in atm getting a major tune up up and service so it should be rocking when it gets back my engine is ment to rev to 6800+ plus a 3.00 ratio in the back end