longer track

Not sure what this hooks onto, but I would question power, gearing, clutching for this much track.

The extensions on a 136 suspension should allow a 151 without a skid adjustment. I would question whether you want to move the skid back due to excess steering effort and heavy in the nose. If you chose to use the 159, your skid would need to be moved back 4" (half the difference of the track lengths [159-151=8 / 2 = 4"]).

Yes you can move the skid back to get a 159 under it but you will run a very big risk of stabing the track i wouldn,t recomend it ....if you want a longer track ....it would be best to find the proper skid .....rail extentions will bend if you land wrong off a jump

bending

rails will bend if you land wrong off of a jump also.......

Putting extensions on the rails adds a lot of extra leverage moment behind where the rear torque arm connects to the rail. The torque arms are a LOT farther apart on a 151 skid than they are on a 136 xtra 10 (they are farther apart then even a 144 skid). They make the torque arms farther apart for a reason. As the distance increases from the point where the torque arm connects to the rail to the rear idler wheel axle, the leverage moment increases exponentially. Also, a 151 skid has a way better suspension design with a lot more travel.

I've seen rails that were bent from having extensions. I've never seen any rails that were bent that didn't have extensions (not saying it's never happened). I personally wouldn't extend any 136 to more than 144. Considering the decent demand and resale value of a 136 xtra 10 vs what you can get a used 144 or 151 RMK skid, I wouldn't bother extending an xtra 10.

Putting 7.5" extensions on a 136 xtra 10 is a lot like putting a long bed truck on a short bed chassis IMO.

I figured there was some "free" parts in his equation some where.

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good to hear that it worked for you. That 500 is a pretty solid engine, I had around 10,000 miles (3 sets o pistons and one crank) on mine when I traded it for more parts for other sleds.