The bell housing and clutch should interchange, the original application for the 435
will determine what adaptor you need to buy/ modify to make it work with the Dana 20.
Is there a NP205 t-case still connected to it? Its not as low of a gear ratio as the D20 but its much stronger. I think certain 2wd NP435 can be used on a D20 without an adapter. But thats just what I've heard... I went with a 79 Bronco NP435/205 to keep cost down.