I'm building an outdoor planter box(about 6.25"x6.25"x13 feet)that will
attach to the side of my house. The pine lumber was nominally 1x8,
ripped to 6.25" and with a true thickness of 3/4". How should I select a
screw or nail to attach the 7.75 x 6.25" end cap? I only have 3/4" to
stick the fastener into, which means that there is only 3/8" of wood on
either side of the fastener.

The concern is splitting. Number 7 screws, 1.5" cause splitting, even if
I predrill; #6 screws can work if I predrill everything and am careful
to make the pilot holes perfectly parallel to the face of the lumber.
With about 40 fasteners required, this is tedious.

The original box was held together with 1.5" staples. I don't own a
stapler, but renting one is a possibility. Any suggestions for nail sizes?

Thanks,

Ray