Looking at putting an EJ22 into my Manx but need to modify the sump. Has anybody got the plans for this modification? Is it better to do it in alloy or steel?

I'd say steel if you are going offroad. Custom Veedub in Brissie do a shortened sump for them that is very good. look up their site. it's like 2 inches thick and has internal tubes and all, but it's not cheap.

here's what i've got,

http://www.geocities.com/peterwood7311/buggyproject/buggyproject17.html 

scroll down to July 18th, 2007.

I have to say, I am having dramas with leaks though.

have one of these on my ej22 T3 van, they are well designed and built and hold the stock qty of oil

By 2" thick, he means 2" deep.
Sharpbuilt also sell them.

Heres our's which is the shortest sump available , and still holds the standard oil capacity http://www.customveedub.com.au/store/index.php?act=viewProd&productId=20