I tell you what, send me your email and I'll send you all the pdf files I have on this engine.

if your willing to tear the motor down and do machine work, you can probably get the displacement over 3.7L and if you sleeve it probably go much larger. the cylinder walls are very thick and are aluminum with a carbide low friction coating or something of the sort. I would preffer cast sleeves myself.

LA Sleeve can do any custom sleeve you require..

as for the cylinder decks, I seen a photo of one of our forum members, she had the dealership install a new engine and the dealer took photos of the problematic original bare block torn down, it was really thick open deck. but yeah I guess you could make a support piece for the walls and weld it in there