Maybe heretic is not the correct word, but a twin turbo Buick V6 with a pair of intercoolers might get you banned from this site. Think outside the box, I hate the phase but it may be 265 cubic inch engine that breaks the box wide open.
A bit of repackaging might be needed for the turbos and intercoolers but the basic V6 Buick is a fairly small package and at 295 pounds (134kg) with alloy block and heads, its not all that heavy and may actully have the same front to rear weight distribution. I think you may need to upgrade the English diff in the rear but I am sure that Spyder could help you there and I don't think making power would be much of an issue even naturally asparated it would scare you.
Basic dimensions are from the Dave Wiliams engine weieghts listing are
Engine - - ---- - - Weight - Length -Width - Height
Buick 3.8 V6 - - - 367 - - - 22.0 - - 24.0 - - 24.2 - Circle Track, August 1994
Ford Kent OHV -- 260 - - - 21.0 - - 22.0 - - 24.0 - Fast Car, February 1992 (British)
Length, height and weight of the Kent are about the same as the Twin Cam, not sure width but probably a little bit greater than the Kent. both engine weights are for Iron blocks and in the case of the Kent and Buick V6, Iron heads.
Bellhousings are available for the Buick V6 to T5 gearbox and with the Chevy S10 shifter and tail housing it should be just about a bolt in.
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