who wouldn't want one of these... and yes, your assumption is right, the 3l has bigger valves, nothing you can't fix on a 2.5 tho.. sure - it would be easier to combine a 2.5 and a 3.0's engine parts, but either way - you're in for a bit of machining !
i *think* the short cut method would be to rebuild yourself a 3.0 motor with a 2.5l crank and off-set the rods to get the compression back up. might like to ask the experts around here. Of course, most people dont bother and go straight to the 3.0.. who knows? mayb one day you'll find a nice 3.0 with an undersized crank? = project time !!
but first, i would consider what my objectives are exactly, you should know that the 2.5 12v is capable of plenty of high rpm power.. just ask jules, that guy has pulled around 250hp outta it. 100hp per liter.. not bad..
the 2.8 is meant to combine most of the torque of the 3.0, with the revviness of the 2.5, with a sound totally its own.. it would be an interesting project indeed.
of the tensioner, i wouldn't worry so much. check for leaks and aim to replace the belt more often - such is alfa ownership. I only personally know of one person who had his belt slip and should have lost his engine - in the end, he escaped significant damage. The majority of others around these parts haven't had any big problems with it AFAIK. I dunno, chasing this conversion for 20kw and to escape the original hydraulic de-tensioner seems a bit of over kill. Hell, us aussie alfisti have known no other device for the gtv6, and i dare say those remaining in sympathetic hands are still going strong.
of gearing, for the street, for me there is no other option than the 2.0 TS gearbox from the 75. Simple <$300 single plate replacement clutch, nice ratios (off the top of my head 2.88, 1.72, 1.23, 0.95, 0.78 ) so it has nice close ratios (same as the 3.0 milano), and of course a clutch type 4.1:1 lsd. but it does come down to taste of course, some *like* the cruisy 3.55 ratio of the 3.0, and some would prefer to pay for the original twin-plate... whatever rocks your boat. personally i love the longer first and second gear of the TS box, real useful for the revvy 2.5, and i think the stock motor doesn't have enough pull for the long legged 3.0 box, he'l not even the 3.0 does IMO.
good luck with whatever you decide.
