Hi Wayne,
All I can say to this is “ouch” … but it goes to show how different people can really be in their needs and preferences 
I’ve attached the charts as attachments to your post (the forum doesn’t allow Firefox to upload attachments for some reason so I used Explorer.
Don’t forget that polyfoam in 1.8 lb density can come in any firmness level and these are only the choices they offer … not what is available in the market which is a much larger range. Density and firmness are not connected and any density of foam can come in a very wide range of firmness levels.
ILD (or IFD when talking about polmfoam which is basically the same thing) is measured as the force it takes to compress a 50 sq inch compressor foot into a 20" x 20" 4" piece of polyfoam to a depth of 25% (1"). The force it takes to do this is its ILD and is used to rate any foam that comes from the same bun or section of foam regardless of how thick or thin the layer is. this only says how firm a foam is when it is compressed to exactly 25% of its height.
Latex ILD is measured on a 6" piece of foam so it is compresses 1.5" which takes more force than compressing a foam only 1" even though it’s not linear because of the different thicknesses. This can result in about a 20% difference or so in ILD (latex being softer at the equivalent ILD if it was measured the same way as polyfoam) although this is not exact because their compression curve is also different and because of the other factors involved.
in addition to this … most people don’t compress a foam to exactly 25% and every foam has a specification called compression modulus (or just modulus or sag factor or support factor among other names) which is the rate that it gets firmer as you compress it more. It is measured as the ratio between 65% ILD (the force it takes to compress the foam by 65% of its height) over it’s 25% ILD. 1.8 lb polyfoam would have a compression modulus in the range of 2.1 (meaning it takes about 2.1 times as much force to compress it to 65% than it does to 25%) while latex has a typical compression modulus in the range from the high 2’s (say around 3) to 4 depending on the type of latex (Dunlop is higher). This means that latex can start off softer with initial compression and then “catch up” and get firmer than polyfoam with deeper compression and has less risk of bottoming out even in softer ILD’s.
In addition to this the elasticity or more specifically the point elasticity of a foam and its ability to conform to a body shape and more evenly distribute pressure also plays role in how soft a foam feels. This also means that it can sink in more in areas that have higher weight concentrations because the response of the foam is less affected by the resistance of the material around the point of compression
Finally latex has a higher resilience (stores energy and bounces back) and lower hysteresis (absorbs energy) than polyfoam which also changes the feel and response of the foam.
All of these as well as some other differences in the properties of each foam (such as how they react in directions other than vertical or up and down) is the reason they perform and feel differently and are not directly comparable to each other based on only a single ILD specification.
Having said all that … I would probably tend to choose a similar ILD to the polyfoam that you are replacing (as accurately as you know it) even though it may feel softer, more pressure relieving, and more supportive at the same time.
You can read a little more about the different types of latex in post #6 here but in general blended is a little more durable (although this is more in lower ILD’s than the ILD you are considering), less costly, and more pressure relieving and has a lower compression modulus. “All natural” is … more natural … and more expensive and a little more supportive (higher compression modulus).
In a single 3" layer there wouldn’t be a lot of difference because you would be compressing the foam more deeply no matter which side it was on. Perhaps a little.
I would choose any of the 100% natural Dunlop layers that are offered by the retailers or manufacturers in post #4 here. They are all good quality and closely comparable IMO.
Latex needs a good quality cover (not a thinner cover or mattress protector) to protect it from ultraviolet light and ozone, acids, and heavy metals which can result in it’s premature oxidation and breakdown. The previous list I linked has some good sources for covers as well. Most people would probably use a good stretch knit cotton or bamboo cover (not just a thin terry) so that it affects the feel of the latex less but you could also choose a wool quilted cover if you were comfortable spending more and wanted the temperature and humidity regulation of wool as part of your system although it will increase the cost of the cover quite a bit.
If your current poly/cotton cover is in good condition and is quite thin it would probably be fine as well.
Phoenix