rhylsadar

hi

there are several savage conversion threads in this forum here. i suggest you read those carefully.
the "problem" you MAY encounter is not really the missing power of that motor. its basically a 15XL feigao like you mentioned. such a motor would "usually" be used with something like a 8s setup.
your basic approach is imo absoluty correct: go for high voltage and a low rpm motor. this will lead you to a powerfull but "efficient" setup. the 6L motor mentioned above is totally unusable in a monster truck like the savage.
i don't read good things bout the esc you suggest though. the turnigy sentilon 100hv may work. check rc-monster.com! BUT there is a big question mark in using airplane esc in cars because you need to brake with servos and only use "forward" on the esc. multiple speed gearboxes do not tend to live long with brushless setups! i highly suggest to fix the gearbox in one gear. you may use the gears of the flux when it comes out. as far as it looks today those gears will fit not "normal" gearbox. the other possibility is the gearbox mentioned above by kershaw.
the lipos you suggest are absolutly on the lower limit. i would suggest to go at least with the zippy H (or rhino) and there 2x4s. so you are high with the voltage and therefore need less ampere to get the power you need for the truck.
however if you fix the gearbox so you can run reverse and brake with the electric motor you need a real car esc like the mamba monster or others. for 8s you may look into the MGM, schulze or kontronik esc.
clearly that wont be as cheap as your proposed setup. but in the long run it will most probably be more fun.
try to target around 35'000rpm as a first starting point. you can deviate from there. higher quality motors (lehner, plettenberg, NEU) can also be run at higher rpm.
so a 8s setup with the 15XL is nice. but then the esc will be quite "pricy". if you go with the mamba monster (lower price) you can only use 6s which means you may use slightly higher rpm motor.
a low cost setup would probably be somethingl like this: fabricate your own motor mount and battery holder, buy the flux gears to fix the tranny, mamba monster, 6s zippy h (or R if they get them again) or rhino with at least 4000mAh and a motor that does around 1500 rpmv, XL size.

i run brushless savage conversions since 2003.

bye
rhylsadar