Good Morning!

Welcome to the site sharpsjt!
A lot of 318 cars came with the 7 1/4" rear so that won't be a problem unless you like to abuse the car and try to hook up with big tires......
The 727 is definitely longer so a different drive shaft or shorten yours is in order.
To mount the engine, use shumacher conversion mounts or find a V8 k member '68 to '72 or if you want to go with spool mounts, a V8 k member from '73 to '76 with the bbp disc brakes would be the way to go in the long run. The factory 9" brakes you have aren't very good for a V8 car and are barely adequate for a /6 car. At least convert to 10" drums.
Don't forget to upgrade your torsion bars and cooling system too.
Why not use a V8 904 behing the 318 and you will not have to worry about driveshaft lengths?
Actually, the best way to do this is to find a donor car that has all of the parts you need to do this swap. You'll be money ahead in the long run, believe me!
Actually, i have a soft spot for '69 Valiants, i've got 3 of 'em.
How about some pics? We love pics!