My first power hammer was a 50 pound Little Giant. I took the rebuilding course offered by Harlan "Sid" Suedmeier in Nebraska City, Nebraska. Check out http://www.littlegianthammer.com/.
I added the usual modifications, spring guard, flywheel brake and a die carriage. Originally the die carriage was a single socket off to one one side of the combination die that would accept a 1" square tang. A variety of dies were made which fitted into that retainer, flat texturing dies, stops of thickness from about 14 gauge up to 1.5", and spring dies. It was the spring dies which gave me the most trouble with wobbling and loosening and loss of control and led to adding a second tang socket on the other side of the power hammer die.
I was very impressed with the increased stability and eventually modified all my dies to the new platform. Later I learned to make my own power hammers based on the Ray Clontz style 50 pound spare tire hammer (STPH) idea. The first one followed the set of plans rather closely in most aspects but after some experience with it I built a couple more hammers with wider throats and 75 pound hammer heads.
On the newer hammers I constructed quick release dual tang die retainers. Refer to the diagram and images above. The basic concept is to hold the die in place with just the weight of the square tube sleeves surrounding the post sockets which receive the die tangs. The die is dropped in place and each sleeve is lifted and a forged "T" shaped key slides in over the die and locked with a large cotter pin which is tethered to the key. To change dies I pull the cotter pins and place them in drill holes in the base tabs and slip out the key and just drop it. The tether prevents the keys from getting separated from the system.
This mechanical system restrains the dies rather tightly in front-to-back and side-to-side directions but permits some up and down movement which doesn't seem detrimental and may actually protect the die. In my situation I found that the stop dies don't require dual tangs but all others are designed for two tangs.
I made a jig to hold the tangs in place while welding so I know they will fit the sockets well. Also the tangs which are made from 1" 14 gauge square tube scrap pieces about 1" to 1.5" in length are "tapered" by collapsing the sides on the leading edge of the tang with the cross peen of a forging hammer.
Stamping each die with some alpha-numeric label is one more detail which aids record keeping. Naturally the stops are stamped with their thickness. Others are stamped with something like BT6 for the #6 bottom texture die. My project notes will record exactly which die was used.