I'm sure a lot of guys are guarded about their powerplants and their sources...however, I find it hard to believe that nary a soul has offered up a suggestion for this request.
There are a few online services that cooperate with salvage yards across America. Try a google search using "parts search" and you should get a plethora of results for these services. Some of these charge fees, some dont.
Personally, I would search every salvage yard within a reasonable driving distance. You might be surprised about the likelihood of finding a diamond in the rough.
I was strolling through the engine building of a boneyard one day asking about big blocks....when one of the yard guys said that he had his eye on a gem...but didnt think he would ever get to the project. I inquired further....
He points to a big blue behemoth in the back corner of the room. Come to find out, a 71 eldo was wrapped around (literally) a pole - leaving only big blue (literally). It was obvious that she hadnt gotten very far before the chance encounter....she was very clean...and intact.
The moral of the story was that after breaking it all down...it was in fact, a very low mileage, complete 500, right out of that car (er horseshoe)....the yard had been saving it for a special customer, then the yard guy, then cash-in-hand prevailed.
Stick with it...there has to be some in your area. I have found a couple here and there since to snag pieces and parts off of. You may have to settle for a piecemeal approach as the rodding business has taken a likin to the big caddies of late. Good luck.
if you are still looking , I have a 500 from a 70 eldo that is for sale cheap and it runs well . Iwas saving it for my 68 eldo if ever needed but had the 472 rebuilt instead
My email is email@example.com