You won't find a HQ 308 with TH400, as I said maybe only an Engineering hack used to make sure it all fitted and worked. As far as I am aware a HQ TH400 sits in the same spot behind a 350 as it does in a HJ so most of the bits would be designed anyway (unlike a HK-HG where 253/308 and 350 see their boxes move due to the longer 350).

I think you'll find the HQ trimatic was at its limit behind a HQ 308, and with the significant extra power and torque in a HJ 308 the trimatic was not up to scratch. HJ was designed for a 400 too so most stuff would have been ready as well.