Best place for any filter is at output of valve body before it returns to tank. Some pressure is available to feed the filter and it won't increase cavitation in the pump due to supply restriction. Sure the old M isn't going to see a lot of hours so longevity isn't really a concern but cold oil...
We used to take a female hose fitting and braze shut the stub end that goes into the hose to us as caps and take a hose to pipe male/male fitting and put a pipe cap on the pipe thread end for plugging hoses. Not elegent but does the job. make as needed and you end up with quite a collection...
I've seen some shears from various manufactures that can cut flush to the ground. personally I'd go with a shear primarily for safety. With a saw you have a spinning blade or chain and debris can go where you don't want it to. With a shear there isn't any flying debris. More controlable. Also...
I just went through all that. I replaced the blend door it is a huge project you have to remove the dash board. There is a repair kit available either way it is a 8 hr shop time project. Something a do it yourselfer cant do. Before you go that route try replacing your radiator cap. If your lucky...