I currently have the external ram but still using the cable on the inside. I want to upgrade and I would like to hear from guys that have converted to full hydraulic.
Did you just buy the helm and the lines and it was that simple?
Or am I missing something?

How does one go about figuring out what size helm is needed?

Call someone like CP Performance and they can help you out.

Definitely worth it, you've already done the hard part putting the external stuff on. The internal stuff is a piece of cake.

You can get the inside kit from CP for about $1500. What you neeed is:

Char-lynn valve
Helm adapter
Hoses.

You can find the char-lynn valve and hoses a little cheaper from a place like www.surpluscenter.com or Ebay, but I haven't been able to find a helm adapter anywhere but CP, at least one that doesn't look like it belongs on a tractor. The char-lynn valve I just bought is model 211-1007-002. You can get different valves that push more or less fluid, translates to more or less turns from lock to lock.