They are internal, but they are adjustable if you know how (Information on rebuilding the steering box can be found in the Chassis Service manual for your year car). If not there are specialist on line that will sell you a rebuilt box off a Corvette that will bolt up to your Impala set up for your turning radius (stops set to keep the tires from hitting the frame). Keep in mind that GM went to a metric fiber rag joint in 1976 so any box from a later car will need an aftermarket adapter to bolt the steering column to the box.And if going to a different box, keep in mind that different vehicles sometimes required different internal stops that will effect turn radius.
Jeep division of Chrysler uses the same Saginaw box that Chevy uses (Jeep used to buy tranies and engines from Chevy as well so there are a lot of parts that interchange even though Jeep made their own head). Additionally Ford uses Saginaw steering gear boxes on their intermediate cars.I used the steering box out of a 97 jeep GRAND Cherokee, 3 turns lock to lock, handles great, stops are perfect, had new hoses made which I had to do anyway since I'm LS swapped. I'd never go back!
There are adapters to go from the SAE or "English" steering boxes to mate to the newer metric boxes. Different spline count and shaft size is what you are compensating for, and this is a very common upgrade. Same for the hoses. You have a Compression Flare fitting on your old SAE box and newer Metric boxes use O-rings to seal. There are adapters to mate your old hoses, or the best solution is to have new hoses made at NAPA to match length and what you have for fittings.I also have a 67 Impala and would like to get the Lares box from Rockauto . 3 turns lock to lock, but it says you need a different coupler rag joint, But it doesn't say what one. Also do the stock hoses fit this box, and is it the same outside profile. My headers had to be formed in for clearance around the box (very close). Rockauto doesn't seem to have tech support to talk to. So any input would be great.