I've used the Prestone backflush kit that sounds similar to your Peak kit. It seamed to work just fine.

I've also heard of people putting Cascade or some other dishwasher detergent in to help clean out the dirt and rust but I wouldn't be comfortable doing that myself.

PlymouthSpDeluxe ,
make one , i did years ago , i think i saw the same  manual you did , and decided i would try aznd make one , i did and it works , i have used it on my own cars old and reasonably modern , also on friends cars with great results , sorry i cant post a pic of it as with my lack of computor skills plus i dont haved a printer scanner, i used a short length of galavanised pipe (only because that is what i had in my workshop you could use copper )i welded a fitting in one end to fit the radiator hose , and in the other end a fitting to take the garden hose , part way along i brazed  in a old tyre vave (that is after pulling it to bits to remove all the rubber seals )so that i could add air while the water was still running through . just be carefull and do not use to higher air pressure , i have used (very carefully up to 20lbs). i have personally never had to use any additive , also you dont have leave the old radiator in , just make sure you can route the water so it doesnt make a mess of you engine bay.
Rex

I feel a tad foolish posting this because it is probably a basic topic for 90% of you, but I would like to hear opinions about flushing out the cooling system.  I got a nice used radiator that's been tested at a radiator shop, so I am not concerned about cleaning it.  I plan on flushing my '48 Plymouth flathead with the old radiator in place, and then swap out with the "new" radiator later.  My purpose is really to flush the block.  It is purely maintenance as there is nothing wrong with the cooling system as far as I know.

In the old Plymouth service manual it shows a great "flushing gun tool".  They have two illustrations showing how to use it to flush the radiator, and then how to flush the block.  For flushing the block they say to remove the thermostat and hook up this flushing gun to the hose that is connected to the thermostat housing.  Open both drain valves (radiator and block).

Now that it is almost 2010 and the flushing guns of the 1940's are who-knows-where (not in my garage!), I was wondering if I could replicate it somehow.  Forget about blasting air into the water since I do not have a compressor.  I was hoping I could use a garden hose to reverse the flow of water through the block. 

I found a "flush and fill" kit by Peak that has a T-connector you can patch into a heater hose.  Their instructions say to connect it to the hose that goes between the firewall and the block.  To back flush the block run the hose water while the engine is idling for about 5 minutes and then continue to flush until water runs clear.  They say to either disconnect the upper radiator hose to let the water spill out, or install a plastic pipe extension where the radiator cap goes (I don't think this pipe will fit on the old radiator anyhow).  I figure I could just open the same two drain valves the old Plymouth directions give.

Will placing the modern T hose connector into the heater hose give a similar flushing action at the old flushing gun tool?   Should I bother with it at all?  Perhaps I should just fill the radiator, open the two drain valves and flush the system repeatedly?  This is all I ever did in the past.

Lastly, should I use any commercial radiator flushing chemicals?  I don't want to risk having modern chemicals do some damage to old gaskets.  On the other hand, if they help flush the engine without harming it, then they would be worth using.

If you want me to post any of the above illustrations, I can try to scan and post them.

Thanks!