727 in a 65 valiant

A 727 can be made to fit the standard tunnel by beating the pinch-weld flat. Of course, it would have to be a small-block one, and easiest if a cable-shift type (I have one in the shed). But, it is heavier and robs power more than a 904, plus people say a 904 can be beefed up to support almost any torque and power of a small-block.

But, since opening the tunnel is an option, consider a later tranny like those for Magnum V-8's (until ~2001). That gives overdrive (4 spd) and there are after-market parts. If your 408 engine will use a later (~1968+) crankshaft, its hole will fit the later tranny torque converter snout. If an early crank, you need a custom torque converter or swap out the front parts of the tranny.

There are also driveshaft issues, but I expect you will need to upgrade the rear-end too, so likely a custom driveshaft anyway. Speedometer connection also varied over the years.