When it comes to radiators there doesn't seem to be a lot of choice for the altezza, I found myself in need of a replacement rad and couldn't stand paying £250 to Toyota for a standard radiator when is200 radiators are £70 upwards. Anyhow, on further inspection, the differences between the altezza and is200 rads are:
IS200 radiator 2" inches wider (fits body ok)
Upper hose connection - about an inch to the right of altezza
Bottom hose connection - opposite side of the rad
Fan shroud fixings - mostly different places
Subframe mounts are the same as are the top clamps. Also the temp sensor in the bottom of the rad is also the same.
Did a quick step by step below, hopefully usefull to someone somewhere as there doesn't seem to be much info on this. In short though, you need to make mounts for altezza fan shouding and also re-route the bottom hose.
1. Comparison between the IS200 radiator and Altezza. This is a KOYO IS200 radiator from Eurocarparts, cost me £90. The top is my old altezza rad, you can see the mounting holes which the altezza fans bolt up to. I managed to use the 2 left holes and made brackets for the others...
To seat the altezza fans onto the lexus radiator you will need to cut off 2 of the plastic mounting points. I have marked these in the picture. They foul the outlet positions on the new radiator.
Mounts cut off:
Next up was making a couple metal brackets to bolt the fans up to. Maybe you could use the is200 fans, im not sure, but I chose to use what I had and make fit. I bolted the 2 left points up to the is200 rad holes, then made another 2 mounts.
Once that's all bolted up securely that's the worst bit done. Next up is the bottom hose. As you can see from the pictures the outlet is on the opposite side. I bought a 32mm (ID) 90 degree silicon bend to attach to the rad outlet (blue in picture) then a 32mm (OD) metal joiner (inside pipes out of view) to slot into this to connect a straight hose to. The straight hose is obviously 32mm (ID) also and runs straight down the radiator. Then on that I bought a 32mm (OD) metal 90 degree joiner to connect the straight hose to the factory fitted original hose. You will need to cut this hose to fit.
And the finished bottom hose re-route setup:
Heres the hose fitted, just needs securing. I have some P-clips which I will bolt up along the hose to hold it steady and straight. As for the top hose, you can re-use the original with a bit of bending or I bought a metre straight piece of silicon hose to cut for the bottom hose, with the left overs you can cut a piece for the top hose, though I haven't a picture taken.
Hope this helps someone, cost me £90 for KOYO rad, £20 for 1m silicon straight pipe, £17 for 2xmetal joiners and then had the 90 silicon bend already and hose clips. In total still a lot less that Toyota prices plus the rad is slighty bigger too.