Not sure how many boards you have, but if it is quite a bit, I'd put them closer together, especially on the lower layers, just to be safe and have a stable base. Massaranduba is very heavy so just go on sight, if you see the bottom layers have too much weight on them, just unstack and either reinforce or build another stack. Mass. 1x6 is rated for 24" span so 3 feet may be ok, just make sure you line up your sticks vertically. Double up your spacer boards on the bottom layer to give it a sound base. Leave your garage door open when possible. If you have a chance to lay them over your frame for a few days before you install the facing, that wouldn't hurt either.