A: I guess you could but I would not. I think you would be better off using finger joint pine 3/4"x4 9/16".
A: yes it can be done ! if You know how to do the work there is no reason to ask.
A: to answer this question I would have to look at the 2 side-by-side as well as feel the surfaces of both pieces ! reading part numbers means nothing to me . EXPERIENCE does mean it all !
A: there are the same quantities of both in 3 nearby stores I checked.
A: ONLY when the radiator is NOT in use and has NOT been used or expected to be used !
A: I would not use any type of pine for a planter box because of warpage even if you prime and paint it. I would use Redwood or Cedar, it is much better for this type of application and bug do not eat them.
A: only if You want it for a year or less . if You paint it or pressure treat the wood it may well kill the plants ! IF I were building such a project I would use cedar wood !!!! cost more at first and would last many more years !