i have never tried it myself.you might have to remove the engine in the 75 and compare it to the 77 engine,mounts wise and all that,note how the exhaust port is,stuff like that.you might try asking this questions at
http://65.12.209.183/yamahaforum/index.php you will have to join there to post but there is many experts on the DT's there.there is a guy named Endurunut there.he is quite the guru.he might know.there is some guys there that know about the 77/78 DT400 and there are many members there with the 75/76 models there too.i own a 75 model myself.