Singer offer a 3.8l with 300bhp, and a step up from that at 350bhp... just most buyers seem to go for the full on 400bhp 4l.
Plenty of other 'shops' could do you a narrow body car in the Singer style... someone like Tuthill, for example, could build you a perfect long nose narrow bodied 911, with a 3.6l engine and c.300bhp. Would be a lot less expensive than a Singer - c.£250k v c.£750k. You'd also have a lot more room to create something very specific, rather than the 'one size fits all' Singer approach to bodywork... so you could have a pure 912 style narrow body, or the ST look, or an RS/RSR, with any aircooled engine option from a 2.0l up to a full on 3.8.
My dream combo would is a 964 C2 coupe, backdated to ST body work, with a 3.8l engine (c.350bhp & plenty of torque), a 6 speed G50 gearbox (from a 993) and single mass light weight flywheel (from a 993RS). Then a nice 'touring' interior.