I will differ with some of the above information. I have been doing Ilfochrome printing since 1988, and have made conservatively over 1500 16x20 prints, both for myself and others - including prints for museums.
Normal contrast paper (CPS1.K), in quite contrasty and about 10% of the transparencies will print without a contrast mask. The low contrast paper (CF1.K) is a marked improvement and will print about 25% of the transparencies without a contrast mask. While CF1.K is a "professional paper" it can be processed in P-3 chemicals with the following slight modifications.
The P-3 chemicals will cause an increase in color crossover problems, however, I have found that adding 10ml of DEZ additive to the 2 liters of developer helps. Also, increasing the processing temperature (of all chemicals) from the recommended 85 F to 88 F helps. Strict temperature control is imperative for all three chemicals as temperature variations seem to exacerbate color crossover. I use a roller transport processor, and let it warm up and temperature stabilize for at least 45 minutes prior to making any prints.
Fresh chemicals are also a great help. With the roller transport machine, I keep careful track of the amount of paper going throught the machine. A 2 liter P-3 kit is supposed to be able to process 32 8x10 prints. In reality, after you hit about 22, the prints will start to show an increase in red, so that by the time you reach number 32, there is about a 10CC increase in red. You will also find that the developer needs to be "seasoned." The first two 8x10 prints will look somewhat different than the remainder. This can be either a slight color difference or exposure difference.
When working, I try to do only 2-3 16x20's at one time. The rest of the printing consists of test prints. I run test prints at the beginning, final prints in the middle of the run, and more test prints at the end with the knowledge that at the end of the run they are starting to look too red.
You also cannot judge the color of a wet print. The prints must be totally dry on the front and BACK. Yes, a wet back will make a huge difference. The prints will look magenta when wet and if you have a wet area on the back it will look magenta on the front. I know, it's polyester based material, non-permiable, etc., just look at a wet print and a totally dry print, and compare the dry print to a print that is dry on the front and wet on the back - you can see the difference.
The material is also very, very sensitive to color correction changes. I use a Beseler Minolta additive color head with a huge correction range and I can see changes as small as 1 number - and often wish I could correct in increments as small as 10ths. I do not find it nearly as sensitive to exposure changes where you often have to make at least 1/2 stop changes to see a difference.
I also disagree that contrast masks are difficult to make. With a little practice, you learn the density of what a mask should look like. If it is too dense all it really does is increase your exposure time. What you are looking for is a low-contrast negative that looks like it would need to be printed on grade 5 black and white paper. That is , totally transparent in the shadows, and gradually building density towards the highlights. Highlight areas should be thin enough that when it is registered with the transparency you can see the highlight detail.
I use FP-4 for the masking film in 4x5 size for transparencies up to 6x7. For 4x5 transparencies you would have to use 5x7 film as you need a way to attach the transparency to the mask. I develop the film in ID-11 1:1 for 5 minutes. If the mask looks too dense, I just make another mask with reduced exposure time.
If you had access to a densitometer, you could measure the density range and max density of shadow areas with detail and build a data set where you could predict the correct exposure for the mask. But, you'll have to believe me when I tell you that masks can easily be made without benefit of a densitometer.
Actually, the hardest part of making the mask is registering it with the transparency. If you have a registration pin-punch the whole problem is much easier - but, with some transparencies (35mm, and those with small frame line areas) a pin-punch would not work. I just use an 8x loupe and my eyes. When the film is registered, I use 3M #235 graphic arts tape to fix the edge of the transparency to the mask. The tape looks like regular frosted "Scotch tape," but it is not as it does not dry out and can be removed without leaving adhesive "goo" on the film. This is true even after 10-12 years of adhesion to a transparency.
Lastly, an alternative to Ilfochrome is the Fujichrome R-3 process that makes really good looking prints in R-3 chemicals.