Exciting times getting the car painted, hope all goes well.
Personally I'd go for a T4 in a P1800 over a T5, good fit, plenty of power (250hp is no problem) but not sure what the fuel consumption would be, I wouldn't think it would be too bad. Personally I wouldn't do a LPG conversion in a P1800 but it's your car. Mattias Vocks p1800 with what looks to be a T4:
http://www.vocks.se/?p=viewgallery&gid=5.
http://www.volvoforums.org.uk/showthread.php?t=70880