If you are looking for more power your biggest bang for the buck would be a 560 V8 and transmission-I doubt there are many cost effective mods you could do the the 6 cyl. You are embarking on a long process(don't ask me how I know) and if you are not attached to a manual trans a 560 engine+trans would be the ticket. You might want to buy an engine from a driving car or a guarentee since the aluminum engines suffer blown head gaskets and striped head studs if overheated. You will also need to consider a 3.27 rear axle to lower your RPM with the larger engine.
I think the 560SL exhaust manifolds will fit the chassis and give a true dual exhaust.

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Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT