A M117 5.0 liter would make a nice swap into a 4.5 if you find a good one with a bad running engine .The alloy block engine is only slightly heavier than the M130 six too. Thw 4.5 was a USA only engine which was made larger (3.5 originally) to counteract the lower compression ratio and mild cam timing required to meet USA only smog requirements in the 1970's. Everyone else got real performing V8 with the 280SE 3.5's.