changing 6v lead blocks by 3,2v LiFePo4 cells
Storage of a battery system is calculated based on the total of amps per hour of each parallel cell. A quick example:

Suppose that you have 4 batteries 12 v each with a capacity of 1 Ah. If you put all 4 in series you will have a 48v battery pack with 1 Ah of capacity. If you put all 4 in parallel you will get a 12 v battery pack with 4 Ah of capacity. If you do two parallel block of two, you will get a 24 v battery pack with 2 Ah of capacity. 

Current drained from a battery is often expressed as a C rating. 1 C is equivalent to the rated current and therefore discharging a battery at 1 C will yield roughly 1 hour of autonomy. If you discharge at 0.5 C 2 hours.

To conclude, if you want to increase your capacity, you will need to increase the number of parallel rows. However, costs may go quite fast in your case since each parallel row wil require 15 cells in series to get the required 48 volts. Depending on your battery management system (because you will need one for safety reason) you might be grouping them in pack of 5, 6 or 8. 

I can't stress out that handling and working with lead batteries is much safer than handling and working with LiFePO4. Please inform yourself exhaustively on the subject.