a- Am I correct with this assumption that I can drop it in, using the existing trans and intake/exhaust manifold?
b- Anyone in BC have one they would be willing to let go? I'm in the Lower Mainland.

a. you are correct; almost: you will need to relocate the water pump (made by Pre '68 Dave). And, you might have to swap the "front mount plate" from your original engine, if the 261 used side front mounts.

b. place a Want Ad for a 216 in the Swap Meet