8D- 20.76 x 10.89w x 8.64h 156lbs 255amp/hrs

4D- 20.76 x 8.70w x 8.64h 124lbs 210amp/hrs

You see the 4D is narrower then the 8D, but length and height is the same. Fitting two 8D's you have a width total of 21.78". Fitting 3 4D's would be 26.1" or 4.32" wider then two 8D's. The 8D's put out 255 amp/hrs (Lifeline AGM's) and the 4D's put out 210 amp/hrs. Two 8D's will give you 510 amp/hrs total and 3 4D's will give you 630 amp/hrs total or 120 more amp/hrs.

I'm going to use the 6V L16 battery that is 11.64" x 6.95w x 15.73h is 119lbs ea and has 400amp/hrs. Using 4 will give 800amp/hrs at 12v. What is interesting is the foot print comparison between the 2 8D's (452sq/in) to 4 L16's (324sq/in). The L16's will take less floor space, but are 7.1" taller. But-just remember that with an AGM battery you can use them in any position (except upside down) that you want. I have a truck driver friend that has his laid down in his side compartment and they are happily running with no problems. Good luck, TomC