not sure about the lpg..
glt alloys, like these
http://www.volvo300club.nl/images/voorblad/foto5.jpg
arent actually too rare, i've got 2 sets at the mo, look like this on a car the colour of yours..
http://www.5lab.co.uk/pics/tours/pages/tours%20040.htm
the rarer ones that would be more in keeping in your car are these
http://www.volvo300club.nl/images/ontwikkeling/4.jpg
they were the only wheels sold on the mk1 300s. we affectionately call them 'black beauty wheels' as that car is the black beauty special edition, and its one of the few cars that had them. i did actually have a set of those wheels for a while, but i gave them to the head of the 'friends of the volvo 300s'. oops!
any 300 or 400 wheels will fit, but not *40 (ie s40 and v40) or early 850 4stud wheels. there is quite a variety around..
hth :)
Hugh