The main reason for the chevy jag is that the V12 jag is a problem engine, unbelievably expensive to work on, and defectively designed (on top of being a VERY tight fit, it overheats due to cooling jacket design problems). Much, much cheaper to get a 350 in there than fix the fried V12.
Ditto for a 350 in a Volvo Diesel when the diesel dies -- rebuild cost is $5000 or so, not including labor, and even the turbodiesel is marginal on power. Great milage, but 105 hp just won't drag it fast enough for most folks.
None of the above applies to a 450slc.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!