Anyone done this mod and is it easy? I read you can use a 3TJ wheel but does it go straight in?

You'll need to change the dog bones to get the bike to sit properly but I'm not sure if you'll get the TJ wheel into the swingarm or not.

I usually say not to bother this, afterall you can put the money you save into some new tyres or better still overhaul the suspension both of these will get you a better ride and through a bend quicker and more safely.

What about a FZR600 17" rear - anyone done this?

FZR600's use an 18" rear, the ThunderCat, FZS600 and TRX850 use a 17".
You'll probably need the whole back end including swingarm but this is probably going to be heavier than yours. The wider tyre might not work so well with your front 110 tyre (the other bikes use a 120) and the 400 won't put out enough power to use the 160 section.
You'll need to gring some matterial off to get the swingarm in, change the dogbones and maybe space the front sproket. THIS WILL probably cost about £500 ... more than half the value of the bike and make it marginally slower.
Spend the money on the suspension and tyres.

Hmmm, sounds like too much effort as you say - having bought the bike for not a lot of money I will get whatever the best 17"/18" tyres are out here and upgrade the suspension.
We are racing it in the No Budget Cup and have to use Bridgestones so it will probably be BT92. By the way it has a 600 engine fited. Thanks for the help!

Measure up the rear shock and post it up .. it maybe that you can use the R6 unit with minimal mods. It was done on here before and I think it was just a a matter of packing it in some way ... but I'm not sure

I put a complete 3tj 17" rear on my 1987 1wg... It only took a couple of collars made up for the swinging arm spindle and having 3mm turned if the rear sprocket carrier. I could'nt get used to the handling from the wider tyre so sold it all. I could've got a narrower tyre on the rim probably but went back to standard ;o)

All the fzr600's run 18" rears from my experience as said above.

I've heard early cagiva mito rears go on fairly easy and look the part, as well as rumours about early gsxr and cbr rears.

I did a similar conversion on my 3he 600. I put the complete rear of a thundercat on but it wasnt simple. I had to first machine the swingarm pivot to fit the frame then space off the footrests and brake master cyl. And after all that the chain was out of line and too close to the frame. I think it handled better with the 18 inch rear but much stickier tyres are more available for the 17s.
All fzr600 rears are 18s apart from the 4jh foxeye which has a 17

i tried a few rear rims but none lined up properly, the tdm one was close but needed a lot of faffing with the rear caliper as the braced arm was difficult to brace to (i think that makes sense!) so i gave up and brought one! actually the one barry sold to me! and it does work just fine and if anything the 3tj swingarm was a lot lighter than the braced one!
all that was done was change the dogbones to give the ride height required. This was all for a race bike so made sense for tyres, not sure i'd have bothered for the road! There are some good 18" rears aswell!

i have done the r6 reaar shock mod all you need on a 1wg is new dog bones and 2 spacers for the top mount very easy mod and the bike handles much better mine is a 3en2 which is nearly the same as a 1wg