how much do you think it would cost to upgrade to the new carb?

 

sparks is like a 130 to bore the stock carb, 475 for the fcr39.

 

The best mods for a 400ex will all depend on what you want it for.

For the engine....... You can upgrade the bore up to a 426 without re-sleeving. (406, 416, or 426) any of those piston kits (with gaskets) will run about $110. Any good machine shop can bore out your cylinder for about $50. If you are going to port the head any you will also need to get a valve job done. About $30. $25 for new valve seals. The port work will cost upwards of $200 unless you do it yourself... At the minimum port match your intake to the carb boot and exhaust to the header pipes. (smooth out the steps)
I would also suggest a new cam. About $150. I went with the Webcam 450/451 grind...
I would very much suggest that you get the 39mm FCR Carb.. About $500 with throttle cable.. You must pipe it. I like the DMC $500.
If you want to go to the 440 kit without stroking it. You will be looking at about $500 for the kit and machine work and I would suggest LRD Performance to do the work. Also get a K&N and outerware. about $50. I opened up the top of the airbox lid with and outerware from Sparks Racing. $15.
These are just some basic ideas from me and if you want more detailed info just ask...

My 440 is very strong. I did my own port work and valve job. and all the mechanical work. It is not that hard to do if you can turn a wrench.

If you are just looking for a better ride, put your money into new shocks for the front and have the rear rebuilt....

 

I allways thought that the easiest and 'most noticible mod for the money' was in fact the addition of a smaller front sprocket. Sure you loose a little top end, but you'll also find yourself shifting from 3rd to 2nd a whole lot less.