Id say if your going with the idea your kicking around, keep the boss standing on the back. The 2nd idea, he is riding bitch, which no self respecting boss would do
So staying with that idea, you need to do some reposing to make the boss seem a bit more dynamic. Look at
CT's boss, notice how he is sideways, and his head is crooked and all that, those are very simple, yet very effective ways to make the boss seem more dynamic. The way yours is right now, he is static. Not to mention if youve ever road in the back of a truck, youll know that you simply cannot stand that way, or youll just fall over. That and surfers are cool looking
Some ideas to help your boss some more. Something Ive done more then a few times is cut OFF the
TL shoota the boss has, glue a "handle" to it, and sling it around his back with some wire. It looks pretty cool, frees up his hand for either another weapon or just something else, and its still
WYSIWYG because the weapon is still on the model somewhere. It also allows you to repose the arm if you wish. That helps make the model not static, OR allows you to d what I did when I made my biker boss (though I took him apart and remodeled him differently off the bike) you can cut off the fist, reposition it to hold the handle bars, and presto, hes actually driving the bike. All those you can do with no
GS (because I never use the stuff as I suck at it

) If you repose the arm for it to be cooler looking, any shoulder work you do you can hide under some cool shoulder pad.
Heres some picks/help on how I made my bike, incase you want to go back with that idea
Firstly the size
As you can see its a good fit, I find its alot easier to get the size down right if you use a bunch of scrap parts and kindda build it to fit the boss. This biker boss was huge and would dwarf a normal biker boy.
What I did was cut a Chaos bike (A
SM bike would work as well, dunno about Eldar stuff, but again, if you hack em up and use your imagination you can tailor it to fit) up, chopped off the rear fenders, and everything infront of the gas tank. Then split it down the middle and added some plasticard in the middle to make it wider. This is one of those steps you actually stick the parts under the bosses legs and use your eye to see how wide it needs to get. The rest is obvious Orkifying, building out the rest of the bike.
What ever step you use, Ill keep an eye on your thread, as I love seeing what people come up with for biker bosses. The chance to make a really cool mini are pretty good. Good luck, cant wait to see where you progress