Used 309L stainless rod and tigged it with about 300 to 350 deg preheat and a nice slow cool down which I know you did.  I would of let it cool quicker then you did, but slower shouldn't of hurt anything.

Mark,
I have welded a lot of cast iron over the years and have had success without preheat.  Ni has always worked. Short narrow passes with peening as soon as the red to orange color leaves. I drill holes at the ends of cracks and die grind a groove 75 % of thickness on those projects that have only one side accessible. I keep my pass width 1 1/2 times the rod dia. I never weld over 2" without stopping to peen the weld.

I think that the 309L is an interesting way to go. I'm going to try it.

Different strokes for different folks.

Dennis

Mark,

I tried fixing a pair of boat manifolds once with the same problem (notice I said TRIED).  I took them to a friend who magnafluxed them, hairline cracks everywhere, looked like a spiderweb.  The ice pretty much shattered the cast.  The owner and I cut one open, there were also cracks from the water jacket into the exhaust ports where we wouldn't have been able to access anyway.  He's very careful about draining everything now when the weather starts turning cold!

Jim S

ive done a bunch of cast repairs,
preheat is to at least 800 and around 1100 is better. to effectuate the repair i used  oxy fuel with a neutral flame and cast iron rods with ton and tons of solar flux.
the action is kinda a stirring motion with the flame and the rod. lotsa flux.
have a large amount of floor dry available and when the part is done being welded then place in the floor dry. if its real critical preheat the floor dry in a container. keep stirring to make sure it gets heated all the way through. then put the item in the floor dry and cover it with as much floor dry as you have. let it cool until it is ambient, may take a few days for that to happen.
takes alot of fuel and at least two guys. using a regular old kitchen oven to get the part up to 500 is the easiest way if it will fit into one.
i learned this from a great guy and good educator bob adams.
i did this on a 53 corvette exhaust manifold, worth more than my truck at the time, also a 150 year old heat exchanger for a water/wood fired heating system, both had huge amounts of contaminants and had undergone significant thermal cycles and both were repaired successfully. these are just some of the cast ive done and very rarely do i have any cracks to rework. that heat exchanger i had to do three times but in the end it was a success.
my biggest advice is to do it on a cost plus basis and tell them up front its gonna be expensive plus.
dont know if you want to go this route but i thought id put in my 2 bits.

also brazed a bunch but thats for non thermal application

welded with nickel over and over grinding out the last weld until the cast is stable works sometimes as well.

predrilling the crack up ahead and behind the crack is a good idea as well.

with exhaust manifolds or anything exposed to contaminants it doesnt hurt to take the whole thing up to dull cherry to clean out the metal.

on the other side of things i brazed up a proprietary flange bearing mount for an antique tractor and it fell into the water under the cutting table just as i picked it up to put it in the floor dry and by the time i got it out it had stopped bubbling, and to our amazement it held and went into service and has never been back
good luck
darren

http://www.castironrepair.com/    if these guys dont know then no one does

http://www.afrox.co.za/products_services/subsubsubcategory.cfm?CategoryID=668

http://www.twi.co.uk/j32k/protected/band_3/jk25.html

http://www.aluminumrepair.com/hts528.asp  this last one i cant speak to it just came up when i did the search.