A joint like this is a trivial hand operation. On a harpsichord with a (roughly speaking ) 3 foot wide by 4 foot long soundboard, it takes 5 or such joints in very thin spruce. If it were difficult, most harpsichord builders would go crazy with frustration. I'm saying, step one is to realize that there's an easy way to do this.
Like several posters here, I find the #7 ( Actually, I have a lovable #8 bedrock) jointer plane works well. There's almost no wood removal, so any suitable shooting plane would work from a block plane, a #4 or #5, a dedicated shooting plane, or just a nice jointer. So long as bottom and one side are square to each other and the blade is sharp, it'll work.
Here are my "secrets."
1) You have to use a shooting board. This isn't a fancy item. You can simply stack one flat board atop another. 3/4" inch stuff is good. No twist but the beauty of a shooting board is that dimensions and end to end bow are not critical. If you're going to use it again, you can screw the top board to the bottom one. You can find a lot of examples on the internet, just figure it out for yourself and realize that many of these examples lack insight into what's critical and what's not about a shooting board.
2) Shoot the joint a "little bit" hollow. On a joint 4' long I go for about 1/32" for a shorter joint, maybe 1/64" to 1/100". The beauty of this way of working wood is that you have this kind of control.
3) Here's the big secret...though it's dead obvious once you think about it. Make sure the faces of the two soundboard halves are finished perfectly front and back before you start. Make sure your shooting board environment is extremely clean because if there's even a thick piece of sawdust on the shooting board it will make a dent. Shoot the left half ( say ) of the soundboard with the face up. Shoot the right half with the face down. This EXACTLY takes out the angle difference in the two halves, absolutely guaranteeing that the two halves will sit in the same plane...that is, the joint will add to exactly 90 degrees. You only need two or three passes with the plane. But more passes won't hurt anything.
This joint will be invisible even before you glue it.
Finally, lay the two halves together with a batten under the joint. Like a wooden yardstick. Secure the outer edges with battens or just a line of nails. When you slip out the yardstick the two halves will fall together with almost exactly the right amount of perfectly even pressure. Put a heavy weigh on each half. If you put the right amount of glue on the joint and rub it first, there should be only a tiny bit of blue bead so you won't have to mess up your faces with a lot of dried glue removal. Just scrape it off gently at this stage with a fresh, sharp scraper. I've always thought that water at this stage could ruin the joint, so I've disciplined myself to put on the right amount of glue. An idea, BTW, that I got from one of James Krenov's books. You can practice this with scraps or you can practice it with the real soundboard. If it doesn't work out, rip the two halves apart at the joint and do it again.
Good luck. PM me if you have questions.