questions on box 2 18" Q series Fi's

I am tryin to work out a box plan for 2 18" Fi Q's, the box i am plannin will really be 2 seperate boxes. 1 for each 18 then bolted togather, then trim paneled in to look like 1 box. Doing 2 boxes to make it easier to pull the boxes in and out(I am a 1 man build crew) of the van(2000 Gmc safari). Here is what i came up with from RE's web site. Each box will be 25" wide by 34" inches deep by 21" tall wood thickness will be 3/4 inch mdf, port width will be 4.5 port height will be 19.5 and the lenght will be 28.75 to give me a box that is tuned at 28.18Hrz and gives me 6.766 ft of airpace. Do you guy think this box will be ok for everyday driving, I am looking for good spl while maitaning some musical clarity. Let me know if this box is ok or if I should build a slightly bigger box(I could make the box 25" tall) to get more box volume, also is 28.18 Hrz ok or should i tune the box higher or lower? Just lookin for some suggestions.