Stirring up a hornets nest

The only swapping I would do.......is freeze plugs! :D Pull the motor out, pop all the freeze plugs out, pressure wash the whole motor....including every which way it will squirt into the freeze plug holes, and the install new BRASS freeze plugs. Especially the ones in the back of the block. Install a new input shaft seal on the 904 while the motor is out, paint the motor and sock it right back in there. Cheap way to insure those 51 year old freeze plugs don't leave you on the side of the road, and spend maybe $30 tops.