The honest answer is that i'm not entirely sure until i have it all in my hands (several weeks away since Tamiya USA still havent shipped out to my US 'helper' )
From reading the posts over on RCTech, it appears people used the RC-Monster MBX6 mount until they machined the true TRF801 version. A few posts say the MBX6 mount fits fine and all screws on the centre diff top plate align. I exchanged a few emails with Mike Cronin at RC-Monster and he confirmed that both MBX6 and TRF801 mounts from him work fine; he did state the TRF801 was the better option but didn't elaborate on what the differences were (perhas range of pinion sizes ?).
I went with the Kershaw one purely on cost grounds, like you say its a lot cheaper than the RCMonster one ($28 toal shipped to my helper). The best i could do with RCM, shipping direct to me in UK was $25 shipping, and then import duties/handling etc - it was looking like a £50 motor mount using that route.
My only concern is that the guys on RC tech seem to be running a smaller spur, 46T/48T versus Tamiya's 50T, so maybe there is a slight issue with the scope of the MBX6, or perhaps they just chose to run that gearing. there is some discussion about Kyosho Inferno spurs (ST-RR, MP777) but no definite answer i could see as to whether they were a direct fit, needed a bit of modding of the spur slot in the chassis or fitted well against the centre diff.
I'm keeping an eye on the thread over there - probably worth reading from page 100 if you are interested
http://www.rctech.net/forum/nitro-of...801xt-100.html
There are a few necessary mods during build..
boil the suspension arms to soften them
rebuild the CVDs, flat spot the cross pins and use flat bottomed grub screws loctited in
use Losi 5430 bladders in the shocks
If you want to go for it find a trusted colleague in the US to do all the ordering for you, there are incidences of kits being incomplete so any issues would need to be sorted out prior to onward shipping to UK.
I also have the alloy chassis braces coming - purely for a bit of bling, the kit plastic ones are apparently very sturdy.
there's currently a build thread on tamiyaclub forum too, in the final stages now.