I have a 1961 Plymouth Savoy 2 door sedan which originally had a slant 6, but I put on motor mounts for a V8, got me an unoverhauled 318 Poly, and was lucky enough to get a "short engine" of the same type, with new heads incl. valves.
Then I got a newer Pushbutton Torqueflite, without the P - brake on the rear, so I put the P brake from a 62 Dodge limo on it, but now some say I cant use this trans, because it has a flex plate, and that will not fit the 318 Poly from 60/-61.
Can anybody confirm this or give me some info about the problem - please.    

The '57 - '61 318 Polys bolt to the same transmissions a 392 will, and require the same adapter(s) to make mating it to something a little more modern happen. The '62 - '66 ('67 truck) blocks don't need the adapter, as they're identical to an LA motor back there.

You may find these links helpful...

The Polyspherical Engine association (there's a list of places to find parts & info):
http://www.geocities.com/polyman64/

At the top of that page you'll find the following links.

The 318 Poly Yahoo Group (which is where we all are (ask us ANYTHING):
http://autos.groups.yahoo.com/group/318poly/

And the Poly Forum, where we'll all lurkers, and casual posters:
http://www.diamondbackengines.com/forums/index.php/board,6.0.html

More info at the sites above...

~Jason