Pretty much the only glider I can think you could possibly fly is the Schweizer 2-32. Not a 2-33.
I do not think there are any current production gliders that can handle 300# in a seat.
Most European gliders are certified to 110kg (242 lb US) per seat and a max all up weight of non flying parts that your 300# would max.
Not only is max GW an issue, but also CG.
Even if you would violate the Max wt per seat issue, our Grob 103 max useful load is 400#, so your instructor would have to be below 100#.

As for the Pawnee, I'll have to check my plane, but I think our Max pilot weight is below 300# for CG limit.

BillT