Do NOT clamp lengthwise. Don't even bounce the bottom piece on the floor to 'seat' the two together. The points of the splice can act like a wedge and split your billets before you can say "Oh $#!^". Don't ask how I know
Take your time to fit them up as nicely as you can, so that the joint is well mated when firmly slid together by hand. Then clamp with moderate side-to-side pressure only. I use two small c-clamps. Proper mating is what makes for a good strong finger splice... not force.
I've never pinned one. I've done bunches of them... no failure yet.
Some folks steam them, clamp them together without glue and leave them cool/dry... which helps them fit to each other, the disassemble, glue and reclamp. I find it faster, easier, and makes a better joint if I just do my best when sawing them out.