I have a 65 Barracuda with a slant 6 - three on the tree want some transmission/engine advice

The easiest and cheapest would be to swap an A230 3 speed and floor shift linkage into your car. Assuming an A230 will just bolt up to your bellhousing. A230's are cheap and strong. They had good shifters and linkage. The rest would be to find the right length drive shaft and get a manual trans floor tunnel for your Barracuda. Only cut the minimum to fit the floor tunnel and do not cut the cross brace. As mentioned above you may have to change the speedometer gear and get a 66-67 A-Body speedometer cable. A V8 swap adds a lot of extra parts that are special to Early-A V8 cars that might be hard to find.

http://www.brewersperformance.com/proddetail.php?prod=FT732