How difficult is it

The hole in the back of the crank on the 1969 engine is a larger diameter than the 1965 engine. This allows you to install a 1965 torque convertor into a 1965 crank, but a 1969 convertor will not install with a 1965 crank. When you use the older smaller (1965) torque convertor with the 1969 engine's crankshaft you MUST use a adaptor/spacer. In other words your 1969 trans will not attach to your 1965 engine without some serious work. Here is a link to a thread about the crank adaptor that should be between your 1969 engine and the 1965 torque convertor. If it is not in place the convertor just kinda sits on the bolts instead of centering on the crankshaft. I believe not having the spacer causes vastly accelerated trans wear.

http://www.forabodiesonly.com/mopar/showthread.php?t=182542