He died of heart problems. If you read the health effects they are claiming many of them seem just normal for a older person at that time. The rest might could also have been caused by chemical issues more than radiation. Heavy metals are for a large part things you want to avoid putting into your body.
For people who are interested in this sort of thing, the TOXNET entry for americium contains a number of excerpts from published work about the case, medical follow up, and eventual autopsy results. The first six case report entries on that page all involve publications involving McC|uskey; look for entries that refer specifically to "US Transuranium Registry (USTUR) Case 246". Because americium is an alpha emitter that principally deposits in bone, it is the bone and bone marrow that are most affected by exposure, and which show the most lasting (and ongoing) damage.
"...Eight yrs after a 64-year old man was exposed to americium-241 in a chemical explosion/, leukopenia was evaluated by a hematologist. Diagnosis of a possible hypoproliferative, myeloproliferative, or myelodysplastic syndrome was considered...."
"...The bone marrow of /USTUR Case 246/ had been substantially damaged by alpha-irradiation from americium, principally on the bone surfaces. A ... finding was a marked decrease in bone marrow cellularity associated with dilatation of blood sinusoids. The severity of these effects varied according to site and was greatest in the vertebral body, where the marrow was almost acellular, and least in the clavicle. In addition, extensive peritrabecular marrow fibrosis was present in some bones, including the rib and clavicle. ... Fibrosis is a common observation in bones irradiated by bone-seeking radionuclides and has been linked to bone sarcoma induction...."
"...The bones examined were the patella, clavicle, sternum, rib, vertebral body and ossified thyroid cartilage; all showed evidence of radiation damage. The cellularity of most bones was reduced, and little evidence of recent active bone remodeling was seen in any bone other than the vertebra, as concluded from the redistribution of the americium in the vertebral body. In several bones, the architecture was disrupted, with woven bone, abnormal appositional bone deposits, bizarre trabecular structures and marked peritrabecular fibrosis. Growth arrest lines were common. When compared with trabecular bone modeling, that of cortical bone in the rib appeared less disrupted. Overall, the results obtained are consistent with those observed in dogs at a similar level of actinide intake...."
In other words, he was 'lucky' that this accident occurred when he was in his mid-sixties, and that he managed to die of heart disease in his mid-seventies. If the patient had been forty years old instead, he likely would have been looking at a cancer of either the bone (an osteosarcoma or some such, and probably at multiple sites if he lived long enough) or the blood-forming cells (leukemia of some sort).