Can I put 400EX plastic on a 300EX's frame???

yes its possible i seen it done before

It can be done, i saw a pic somebody posted here recently (do a search) but the real question is....."Is it worth it?"
And the answer in my opinion is no. Yes, the 400ex plastic is really nice looking but why would you want to look like a 400 and perform like a 300? It costs a pretty good lump of gold to buy all of the parts you'll need....headlights, hood/nose, fenders, sides, tank cover, etc...
Personally i'd rather spend that cash on making it run like a 400ex rather than just looking like one.

400ex plastic is way less than half the cost of 300ex plastic. You can put a 400ex seat and full plastic on a 300ex for about the cost of just OEM 300ex FRONT FENDERS !

Lorrin

I've saw it done before and it looked awesome. I think they had a little problem with the 400ex headlight tho.

You're probably right leel, but again unless you need the plastics i'd suggest spending that cash on something better.
Also you have to add other stuff into that price too, you need much more than just fenders and a seat. There are other problems too like will the gas tank and fuel switch fit under the 400ex fenders or will you need a new tank and switch. You'll also have to find the hood/nose assembly, lights, side pieces, tank cover, and maybe even 400ex brackets.
Still doesnt make sense to me.
I'd like to talk to someone who has actually done the swap to see what it cost them.