I remember reading somewhere that smaller bore longer stroke engines are inherently more efficient. It was somehow related to vintage race car motors (old Ferrari and Jag engines were WAY oversqaure). Smaller engines are also generally more efficient so I'm not sure a 300 is a good starting point for any vehicle smaller than a dump truck. I used to drive a Ford dump truck that had a small (for the job) car-type (didn't have replaceable bore sleeves) engine in it and IIRC it was around 300 CID and while it wasn't ideal it did move the 70,000 lb truck down the road at 70 mph everyday.
Best parts source would probably be dodge/ cummins stuff since it's also inline 6 cylinder configuration and there is a pretty good after market developed around them. Might could even use internal parts, I'm not sure but it's definetly worth a look.
On gas engines static CRs range from around 6 to 1 up to around 15 to 1 but if you look at dynamic CRs there is a MUCH smaller range, ie it's the dynamic CR that sets the limit and this is largely dependent on the cam timing I would assume diesel is the same way but to what extent I'm not sure.
I think it would be more fun to find a way to bolt 2 of them together and make a flat 12 boxster out of it
