Hall Scott made 6 cylinder engines up to 1100 cubic inches,Continental made a 802 cube 6, these engines were generally used for pumps or generators. Mack offered a 707 cube gas inline six, I bet the engine weighs over 1200 lbs.
Yeah a BMW 6 is real nice,probably 5 grand used with the electronics? I once saw a hot rod using the old Jaguar DOHC inline 6,about 260 HP stock, nice vintage looking engine.Or a Toyota Supra had or has a real fine inline, again nasty expensive I bet.Mopar slant sludgers and AMC 6's can make good power.We have a 96 Cherokee, the 4.0 liter(old AMC design reworked by Mopar) inline makes 190 HP, I think you can get the engine ,5 speed and all the electronic stuff for around 2 grand from a recycler or adapt the injection to a GM inline?