Ive seen a few Detroits converted to steam......there is no need for a new cam ,the conversions use the fuel injection cam to open an inlet valve ......In the 80s ,I had a steam truck powered by a 3/71 converted to steam.....just for around the farm...it used a giant steam cleaner I got from the army auctions for a boiler...unfortunately,such things run on kerosine or diesel,and with about 1% efficiency ,arent practical except as demonstrations......If you are happy to run on low pressure,you can make a conversion using the head as is ,steam in through the exhaust ports ,and exhausting via the air box ...preferably with a condenser to generate some vacuum.