Non A-body question.

sure, a 318 will fit in there...but you already know that...all SB's are the same external dimensions.
Important stuff is engine casting date....usually under the driver side exhaust manifold.

A 302' headed 318 is a good little engine with a 4bbl and a set of headers.....so if it is a 1985+ 318, and runs, just pull the oil pan and check for cleanliness, pop the valve covers off and do the same, change the timing chain/gear (all 318's that I have seen have nylon gears and single groove chains) and throw it in....you'll have to plug/weld up the Smog Ports on the 302' heads or you'll have 8 exhaust leaks....otherwise...good luck.

For a transmission, I would try sourcing a non-lockup TF904 out of a
late 70's early 80's 318 equipped pickup, you can use it and the torque convertor straight up, with little to no modifications.