If you are going to pound on your transmission, a C-6, even a stock one with a shift kit is the way to go.
In your case where over drive is required, the AOD is pretty much the cheapest over all option. It'll run you around $2K + to get a unit capable of handling the torque, but its a one piece unit. The E4OD is not really an option, you have to cut up the floor pan to fit it in a 60's car. Its a monster in size and really works best with a computer. The other overdrive option is a GV (gear vendor) overdrive which bolts on the rear of a C-6, its a great choice - but it runs about the same (a couple of hundred less than 2K), but you still need a built C-6 to hook it to.
Fora picture see the link below, its about 1/2 to 1/3 down page, Combined it gives about 20% overdrive, 30% milage: http://www.transmissioncenter.net/c4_c6.htm
There is a difference in cruisomatics, they have a few different sizes, the Large case cruisomatic, put in Lincolns and some Mercurys, was the transmission the racers used before the C-6 was available. Problem is - not too many know how to put them together, to make them last, anymore. They have a tendancy to crack the case if abused.
C-4 and AOD are lighter, don't rob as much horsepower as the C-6. But, my personal opinion, they need a lot more PM ,preventative maintenance, to keep in top shape behind a high torque/HP, street application. Some guys swear by them, but I just can't let go of the big block needs a C-6 mantra. As mentioned Lentech has been building and selling coversions for use with FEs for a few years, they are pretty much at the top of the list if thats the way you want to go.
Bottom line: For the lowest cost, best value when overdrive is required ... The AOD is probably the better choice.
Behind a more robust engine, or more abuse with intent to do harm ...I would say look at doing it in stages. Get a strong C-6 built, then save up your pennies for a GV overdrive unit later on. It costs more, is heavier...but if they can survive behind 40 ft+ RVs, they should be a better lasting value behind a high torque, high horsepower streeter - if you need to take longer trips.
Obviously - these are Opinions only. You need to decide what is better for you - what fits your uses.
<font size=-1>[ This Message was edited by: Beoweolf on 12/7/06 1:51am ]</font>
<font size=-1>[ This Message was edited by: Beoweolf on 12/7/06 2:01am ]</font>