) First: Change from automatic to manual normally M90 should fit the engine or? I know it's a lot of work but for this car I would do a lot 
I heard about a Volvo 960 3.0 with a BMW e34 540i gearbox with 6 gears Anybody here who knows about a rebuilt like this ? By the way my winter car is a BMW e34 --> fucking love the 6 cylinder in both cars 
Secondly: Get more hp out of the 3.0 engine. maybe with turbo or compressor but I would prefer not to boost the engine like this But I let myself be convince by the opposite if there are good solutions I think about getting more cubic capacity and polish the channels. Is this possible ? I hope you had a good start into the new year -) Greets Michael