Need help with configuring backups correctly so I have backups upto 6 months

True Image 2014 Build 6673

Hi: I want to backup a critical computer (the data on the drive is about 800M) and be able to keep backup for upto 6 months. I have a external drive that's 3.5TB. I am running out of diskspace within 2 months. Each full backup is about 680M and the differentials is about 9M. I want to backup Mon -Saturday (5 times a week). What'z best scheme so I don't run out of diskspace after 2 months.

a. Should I do a full backup (fb1) say at the start and just do a differential (db1, db2, and so on) every day for 6 months? How do I set it up so that differentials older than 6 months get deleted.
b. Or should I do one full backup every month (say 1st of every month) and differentials every other day. Again how do I ensure differentials and full backups older than 6 months are deleted. So after 6 months this will have maximum of 6 full backups (6X688M) and 174 (29*6) differential backups (174X9M). This will add upto 6TB of data ( (6*688) + (174*10)). I am guessing I will need to get a bigger drive. Or is there a way to accomplish this with two different drives (example: 2 4TB external drives) with backups being sent to each drive every alternate day?
c. Maybe someother option which is more efficient.

Thanks
Ravi