I don't think Nissan diesel is any heavier than slant six. My Valiant had LD28 engine before I got it. LD engines have been somewhat popular conversions. I have seen these in Valiants, Volvos, TransAms, even in 60s Cadillac etc. One local LD28 Volvo 240 was really hot rodded, it ran high 11's at quarter mile.