Port Design

I am building a box for two 15" DC subs ( SPLW 15's ) the spec's recommend 3.4 ft3 of space, tuned to 40 Hz and they say you need 2 ports 4" in Diameter and 10" long for each sub.  I have built quite a few enclosures in my short career but this will be my first ported.  The speakers can handle 1400 RMS. my ?'s are do Ireally need all that power even though they are only made for ported enclosures and does the box have to be 7ft3 for 2 and can I make the ports slotted into the box and if so how? or should I use those PNR ports or what.

 I am really into car audio & video but have always used sealed boxes and need as much info as anyone is willing to share with me!! 

 Anyone with any experience building ported boxes please help I am really wanting to start on this project but these subs were built only for ported and I am an idiot on this subject (not that I am not an idiot anyway)

do Ireally need all that power even though they are only made for ported enclosures

Check the woofer manual to see what the range is on those woofers.  Most of them can perform fine with half the RMS.  If there is no information about that, contact tech support of the woofer manufacturer.  I've found most of them respond quite quickly to questions like that.

and does the box have to be 7ft3 for 2

3.4 ft^3 for one would make it 6.8 ft^3 for two.  Any changes at all in enclosure volume would necessitate a change in the ports.  For your first ported box(es), it would be best to follow manufacturer's guidelines.

and can I make the ports slotted into the box and if so how?

Most spec sheets for vented subs will give you the sizes of slot ports if that's what the maker intended for the subs.  But I have been getting some use from the free program WinISD.  If you download it, follow all the help files, and get your woofer specs entered into it, you'll find that you can switch from round ports to rectangular ports by clicking on the port type icon.  The program will figure the different size for you.