I have 2qty F cell nimh packs. 13AH ea. Hardly worth the trouble IMO. You will have serious balance issues with 20 series cells and unless you use them regularly they won't compare to SLA AH range. They lose charge just sitting for day or two. Then if not used regularly they just don't avheive max range. Now stuck carrying more nimh batteries than original SLA. Result = no weight savings.
IMO start with lead and save for lithium.