Low blood pressure and dizziness upon standing

I'm 77 years old and 6'7" tall. 250 lbs. Most of my life since reaching full height I have occasionally experienced "head rushes" as I used to call it, which can also be called "light-headedness" or dizziness. For the past 8 months this began happening more frequently until now in the last 2 months it happens nearly every time I stand up from prolonged sitting. I have never fainted from it, but I usually have to put my head below my heart to stop the dizziness. Sometimes it lasts for 5 minutes or so even after sitting down.

Last October I was asked by my cardiologist to track and report to my portal account my blood pressure and pulse. Original reason was that my pulse was getting dangerously slow while sleeping (with a heart monitor on for 7 days it got down to 26 bpm with one pause of 5.1 seconds between beats). This eventually led to getting a pacemaker installed in March. With this taken care of, as well as my getting shocked out of AFIB in February and my heart failure ending a month later, I was declared generally OK.

But, my blood pressure ranges low to severely low. Common for me is 105/80, which is just fine. Not common, but also not infrequent, are bp's of 73/56 +/- 5 for both systolic and diastolic. I have lots of friends who have been dealing with high blood pressure for many years, but I've not known anyone with low blood pressure challenges accompanied by dizziness.

I'm looking for help understanding why the dizziness and low BP. Recommendations?

Thanks.
Bill