The SP-1 lower is a wondeful place to start from. It won't quite authentic as it is a slab sided receiver but it still makes an excellent weapon. The more difficult part is acquiring a large hole flat-top upper if you wish to do that coniguration. You can always use small pivot pin lower and adapt them.
As an another option you can make a SP-1 carbine or a XM-177 clone. There are fake "sound suppressors" that will screw on to a noodle barrel to make it look the the XM-177's.
Good luck, have fun and enjoy.