Pulse Adrenaline 250 Rear Shock Options

Hi there, i Own a 125cc but i'm also buying a 250cc too...
the problem i'm finding is that i'm getting a lot of chain stretch whilst day to day riding
(the 125 runs a 428 pitch chain, the 250 runs a 520 pitch chain)

Will upgrading my rear shock on both make life a little easier?
obviously i'm expecting the 520 pitch chain to last a little longer...

but for arguments sake... are there any known Rear Shocks that will fit?
...i've already killed two rear shocks through street use... (I'm 15 1/2 stone ;) )

Search Dr200 mods in 'net . There was 'o ghetto idea http://item.taobao.com/item.htm?id=44562052082 (truck cabin shock) among dirt riders in China when monoshock aren't easily available