2.8 td gu patrol conversion

hey every one
just a quick one i have a 2.8td with about 220kms on the clock of trouble free driving. but i am a bit sick of lack of the low down grunt, it tows ok , a carpenter it has served me well for towing around my work trailer, i am also about 2 purchase a caravan because i am wanting 2 do some out back traveling as well.

has any one upgraded the engine 2 a 4.2 td, in that pacticular model if so roughly how much would i accept 2 pay for it, & what sort of economy can i get out of it,

i have heard about those 6.5 liter diesel conversions people have had done 2 there rigs but im after something that will be realible & deliver reasonable economy as well seeing that i will be using it for work as well!

thank you