Since your father is 66 years old with a Pre Existing Diseases, it would entirely depend on the underwriting team of the Insurance Company to decide whether he would get Insurance and if yes, then the extent of the coverage.
However, it is still worth taking the plan, since you would get coverage for all other ailments other than pre-existing conditions from day 1.
The concept of insurance is that it covers unknown risks and not known ones. However it will still cover all your ailments after the certain waiting period is over.
The plans that you can consider now are:
1. National Insurance Varistha Plan for Senior Citizen where the Pre existing diseases are covered after 1 year of operation.
2. Star Health Red Carpet Plan for Senior Citizen, where your pre-existing diseases would be covered from the second year of operation.
However, you can also consider normal plans where you also could be a part of the plan maybe. For example Max Bupa Heartbeat Plan where all relationsw can be covered under the single plan but it covers all pre-existing diseases only after 4 years of waiting,
Also, if you wish to meet any representative, please do let us know and we can send people from the respective companies over to you. Hope your query is resolved. If not, please do feel free to get back to us.