There's a lot of if's and but's in my mind at the moment but I think it might be do-able.
It would be a hell of a peaky engine. If you were going to do it you'd have to consider using the 3.0 intake pipes and runners and GTA cams as well. [
] [LOL]
So, valve sizes according to an ebay shop are:
2.5 inlet:
Head Dia: 33.50mm - Stem Dia: 7.00mm - Length 102.00mm
2.5 Exhaust:
Head Dia: 29.00mm - Stem Dia: 7.00mm - Length 102.20mm
3.0 Inlet:
Head Dia: 35.60mm - Stem Dia: 7.00mm - Length 102.00mm
3.0 Exhaust:
Head Dia: 31.10mm - Stem Dia: 7.00mm - Length 102.10mm
The heads are listed as different part numbers which raises some questions regarding compatability.
As the 3.0 is a larger bore, is the combustion chamber too big?
Will this affect the compression ratio?
As the 2.5 is a smaller bore, is there enough room to accomdated the larger valves?
The last question I think I can answer myself as I have a pair of 3.0 heads and I think I have a spare 2.5 piston and liner. Problem is I don't have a 2.5 head to compare it with.