Reply to "V12 Pantera?"

Depending on the all-up weight, it looks feasible from your dimensions. And it can't be heavier than a Ford Triton V-10 (which will also fit, but even though it's a SOHC, its also an iron truck engine). I hope your builder did a good job as some BMW V-12s had weak bottom ends. Remember that blocks up to 4" LONGER than a 351C have been successfully fitted into the Pantera; the halfshafts do not need to run perfectly straight- the ZF can be slid back up to 4".
15 years ago, Ryan Falconer welded up a pair of aluminum 350 Chevy blocks & heads to make his 7-liter 'Falconer V-12'. He had one stuffed in a 'Vette and another in a Chevy pickup at several car shows. The only thing stopping me from going with one at the time was the $25,000 cost. Hopefully you aren't into the bimmer quite that deep.... Let up know how it runs & handles.