You should have bought all four of them and parted them out. Good money to be made. Those body parts are downright rare. Only about 7000 780s were made (just like the 1800ES)

I converted my 745 from AW70 to M46. It shouldn't be any more difficult to do a 780. I remember once seeing a web page about such a conversion. It had a list of all the parts you will need.

The first year of the 780, 1987, had a solid axle (same as a 740) 88 and up have IRS. 87s may have deteriorated electrical wiring under the hood. 91 was the final year. These were called "Coupe" instead of 780, and have a few other unique features. There was an article in Rolling a few issues ago about one of the very last ones produced.