I also used 3/4" ext. grade. If you need to slip it into a "C" channel it will help to use a router and shave off a bit from the edge down to about 5/8" so it will slip into the channel. I also took a roundover bit and gave the channel edge a roundover on both sides. I think it will make it go into the channel easier. Gives the floor a good stable feeling when you walk on it. I also chose to re-enforce the spans between frame rails with a strip of plywood screwed below. Makes a nice secure joint, reduced movement and floor squeaks as you walk on it. I didn't glue it but that would be a good thought.
Roger in NJ
" Democracy is the worst form of government. Except for all the rest"
Winston Churchill 1948
TAC - NJ 18