Yes welding (nodular) cast iron manifolds is tough but the good news is that you should know whether it is successful as soon as it has cooled - if it's going to crack you should be able to see it then: if it's all good then it's unlikely to crack in a few weeks time (other existing weaknesses notwithstanding).
I use NI rods. The biggest issue is keeping the global temperature even across the manifold, so yes the most common way to do that is to heat the whole manifold before welding. VERY important that you then bring the temperature of the whole thing back down SLOWLY. Burying it in a tub of sand has worked better for me than using an oven
An alternative is to weld it cold but super-slowly (i.e. in small amounts, waiting for it to cool in between each little weld) so the local heat doesn't get too high. Tapping the weld with the round end of a hammer in between each time apparently helps it cool evenly. I have tried this way once (too lazy to get the oven
going that day) and it failed, so I'm no expert (and was doing it on second-hand advice so might have got the technique wrong in any case).
With proper heating
and SLOW cooling
my success rate is probably more like 85% - 90%.