forgot to mention, Rover transmissions will NOT bolt up to the 300ci without an adapter.
the Buick 215ci has a unique ( for GM ) trans pattern which was not used for any other engine in the Buick small block / v6 family. when Leyland bought the engine plans and converted it to metric ( 3.5L ), they kept that unique trans pattern through the entire production run up to 2004.
Buick, on the other hand, converted the cast iron blocks ( 300, 340, 350, 231 v6, 252 v6 ) to the slightly more common BOP ( Buick, Olds, Pontiac ) pattern. this is still not the same as a Chevy pattern.
a lot of Rover guys love to scavenge 300 cranks and throw them in a Rover block. it's a cheap and sleazy way to do a massive stroker upgrade to any Rover. even a 4.6L only has a 3.228" stroke, the 300ci is 3.400" out of the box. the only 'production' Rover that outstrokes the 300ci is the 5.0L that went into TVRs and other sporty variants and are rare as hens teeth.
i think one or two brave souls have attempted a 350 crank into a short deck alum Rover block but you start running into interference issues.
this article focuses on the US 215ci mods but should give you some food for thought:
there's also a lot to be found over at the Britishv8.org forums. most of their work is looking for Buick stuff that can be adapted to Rover engines, as you might expect, but it will point you in directions where you can come back the other way.
oh yes, there's also the Australian variant, a P-76 or some such thing? i think that's got a taller deck block than a standard Rover / 215.
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