Health Insurance recommendation for US Senior?

Try any of these. wagonersabroad.com/medical-insurance-spain-us-citizens-non-lucrative-visa-application/

I think that you should take out travel insurance. There are some online comparison websites around. You can compare policies and what they include and exclude. 3-6 months for travel insurance is not too expensive if you shop around. I think there is one call world nomads and the best one I found was called aussie travel insurance or something like that. Sorry I can't find the exact name but it had the best coverage for everything and was the most affordable. Good luck with it.

Usually you sign up for a year in Spain, so you  need travel insurance. Many major US insurers (if you already have a US insurer apart from Medicare) have International offerings now, like Cigna and Blue Cross/Anthem. Look if you qualify for those.
Not a chance you will get insurance here if you have a major pre-existing condition.
If you have been in the military, Tricare may be an option as they reimburse up to 75%.
The aforementioned wagonersabroad is probably your best travel insurance option.
Allianz may be a good option. A lot of German companies control various industries here, including insurances. Allianz is a huge player here.
Another one is also GErman and is called Bupa. They have the biggest Expat experience.
I would consider US companies as second not first choice. They usually are very inexperienced and have little market power here.
DKV (owned by German Munich Re) is also a good quality  major player here. Not sure iif they offer 6 months though. You would have to call them. They are experts in expats and have an English site as well. Good prices and excellent nationwide network, even university clinics.
Axa is another good international insurer but one has to pay attention what they cover and they have all kinds of caps. They have a great website though so one can figure out what they cover and they seem to have good prices.They also accept preexisting conditions which they then exclude and they ask no medical records at application, only at claims time.
One caveat with all these insurers: they offer hospital coverage, but reality is that what counts is outpatient coverage since most of us do not end in hospitals overnight anymore and only need specialists out patient care with major issues. So make sure the last one is well covered.