What size engine in this classic please if anyone can help

Just had my P38 through its last bit of work , the LPG switched back on. O how sweet it runs and for so much less. Sorry but dont know how folks run on p-etrol all the time, its a killer in the wallet dept.
Anway for the fee of tuning her up and giving her the all clear running on gas (had heads stripped off and I fitted a new double vapouriser) my LPG man said Buy me a top end gasket set for his newly arrived classic rangie which sounded very sick. Now I've looked at the Island 4X4 site and I'm confused. His G reg classic has fuel injection. Does this mean its a 3.9 instead of the 3.5 he thinks it is, if so will need different head gaskets, as its a bigger bore , yes or no. Please help so I can help others

Just done a little homework and found out he has a 40th aniversary Baluga black RR classic with a 3.5 engine, sunroof leather and dark wood caps on doors. Rare beast in black apparently