Question on fitting modern trans in 71 Demon

Hey guys,

My son's 71 Demon has a 98 360 in it with a 904 attached. The 904 has been giving us some trouble. Depending on what the problem is and cost to repair, I'm thinking of putting in a modern 4 speed automatic. 518 maybe?

I'm wondering what parts are needed to make it work and if the floors have to be modified? Is a 518 the best way to go? What is the approximate cost? Can I do it with junkyard scrounged parts?

thanks !