46 Post Steering ram

Usually, the AP just need a guide as the volume the existing system uses. Teleflex / SeaStar owns the remaining products and support of the old Hynautic systems. Go to their web site and match pictures for the model #.
The install guides also mention the boat size to help size the AP pump.
I can bet you a cup of old coffee, No matter what numbers come up, you going to order a common Type 1 pump.
What brand AP are you shopping for?

The only difference I can find in the smart pump (2.1) over the regular inboard bump (2.0) is it works with power assisted systems. Both operate 10 to 24 ci rams.
The pump 1 (1.2) operates rams to 10 ci.

Keep in mind, 10 to 24 ci is a large cylinder. I was just reading some reviews where the 2 pump was over-sized and not working correctly and the 1 (1.2) pump was the fix.
Other reviews say the 2 is fine on all sizes but I am reserved to agree with that.

I used to install RayMarine APs. Their plane (low tech and not smart) type 1 pump worked on about everything up to 50 foot fine. I did prefer installing them near the cylinder. I felt the rudder could respond quicker.
So, I'm working on a customers 53 Southern Star, 1300 HP. There on the transom was a Ray type 1 AP pump, controlled by a Simrad setup. Owner reports it works perfectly.

I was spoiled to the ole Robertsons stuff.
When the Simrad AP stuff came available, It turned out to be good stuff also.
I locked onto a AP25/26 setup ten tears ago for our Bert. Keeping the Robertson CR pump originally installed, It has been a great system. Newer AP 28 stuff on other ships work great also.

Don't know anything about the AP70. I'm sure, based on my past history, it will work fine.

Just looked the AP70 up. Looks cool.

Enjoy,
rc

You do not want to splice the signal cable. It will come back to haunt you. There is your max distance away from the AP control box.

Per typical instructions, as low as possible, center of ship and away from bilge pumps, sump pumps, compressors, blowers, batteries, generators and heavy power cables (ACv or DCv). That probably will not leave you with much choices left.

Even these fancy rate compasses still want the most comfortable spot on the boat.

I would start just forward of the engine room bulkhead, below the steps leading forward. Make sure there is nothing magnetic on the engine side of that bulkhead and away from forward bilge or sump pump.

With those thoughts in mind, some Post owners will be chiming in soon with an experienced location.

Reminds me of an AP28 issue.
I added a shower sump in the bilge of a customers boat. Rite next to a current bilge pump & behind a macerator pump.
Did not relize the AP compass was a few feet away behind a hull frame (plastic boat bilge divider).

A few days later the owner was trying to explain this weird AP problem that had just started.

The AP worked with the original pumps, seems the combined magnetic strength after adding a lil Rule 800 was to much.
From a skinny 4 feet to over 6 feet the compass was relocated and works fine now.
I wish I could of moved it more, but no more slack in the signal cable.