Built a few of these back in the day,, Jemco kits,, never the AT-6, but my buddy did,,

Sheet the wings first,, that's common with foam cores,, for this size typically you don't need a joiner spar,, just about a 3" of 2-3oz fiberglass over the center joint,, top and bottom

Control throws?? I always eyeballed those,, for this size I'd say start about 5/16" to 3/8" max with the ail and ele,,, 1/2 to 5/8 rudder,, then adjust more or less from there

These land hot,,, don't try and float it in,, the AT-6 is known to tip stall

good luck