Unfortunately, I know of no such website or roster that provides a "complete" listing. For dome cars, I recommend:
http://www.trainweb.org/web_lurker/WebLurkersDOMEmain/ as excellent reference material.
I would suggest the Amtrak Car Spotter series of books and some others, such as "Car Names Numbers and Consists", edited by Robert Wayner, published by Wayner Publications in New York City. They are out of print, but you should be able to find a copy on E-bay or at a railroadiana show.
I have some internal company roster documents, but they are not a May 1, 1971 listing. Both cars were on the roster in service as of 10/01/1976. They are not listed in the Amtrak Car and Locomotive Spotter (4th edition) with dat as of 4/01/1980.
Amtrak later re-used roster numbers 8380 and 8381 for dining cars acquired from Southern Railway.
Keep in mind that Amtrak acquired additional "used" equipment at times after May 1, 1971, prompted by events such as the 1974 petroleum shortage. The x-US Army troop ambulance cars and hospital kitchen cars and some Via baggage cars are an example of "used" car acquisitions by Amtrak after May 1, 1971.
Keep in mind that there are lightweight passenger cars which Amtrak did not acquire (original Autotrain Corporation, D&RGW, Rock Island, Georgia RR, some cars in "commuter" service, and cars retained by the pre-Amtrak operators) that exist today, some of which are in service. However, you might consider these cars outside the scope of your research efforts.
