wtp over drive trans behind 340 in a 68 a body what can i do

The only tranny you have any real luck with is the GM 200r4 is it smaller then both the 904 and 727. You'll need to get an adapter to bolt it to the block, a new converter and make a new mount bracket. You'll also want to have the tranny beefed up inside (these tranny, when built right, can handle a lot of power. Seen live quite nicely behind high powered twin turbo GN with no problems).