I've checked Yoshifab, STS Machining, and DeeWorks' sites, and none of them seem to have any off-the-shelf kits to do a B6304 into a 164 swap (or 140 either). However, each store has other things you may want to complete the swap.
Any kit which will do the job for a 140 won't do the job for a 164 because longer 164 wheelbase.
IIRC and if this info is of any use, the B6304 sat in the same relative position in 940s and 960s, so it's possible to extract redblock and insert whiteblock in a Seven or Nine by changing the engine mounts on the crossmember. Mister Bear has done such a swap. OTOH, 164s and 140s don't share the same relative engine position, because the 164 has a few extra cm wheelbase. With far fewer 164s around than 140s (which are few now and getting fewer) it's unlikely anybody will develop a retail kit for the 164.
But don't let the lack of a ready-made kit deter you, this is a project which ought to be done. Volvo power is good. Jag går hellre än jag köra halv-uppfödda japan skit! So you'll get to design and make your own engine mounts. You will need to spend some time with your measuring devices, your B6304 engine block on your engine crane, inside your vacant engine bay, some blocks of timber, your drafting tools, your metal working tools, and then your welder.
Certification, you say? As long as you do things sensibly and properly, and you're prepared in advance for the obligatory bureaucratic BS, obtaining the Blessings Of The Gubbermint is not so difficult as some would have you believe. You need to read this -
https://www.vicroads.vic.gov.au/-/media/files/documents/safety-and-road-rules/vsinumber8guidetomodificationsformotorvehicles.ashx?la=en&hash=ADC5076B9F3DA0D047319217ECDEDEB1
....and you also need to look at this page -
https://www.vicroads.vic.gov.au/registration/vehicle-modifications-and-defects/vehicle-modifications/vass-approval-certificate
.... and this page -
https://www.vicroads.vic.gov.au/safety-and-road-rules/vehicle-safety/vehicle-standards-information
.... and then you probably ought to talk with one of these guys or girls about what you want to do, what they need you to do to pass that, what they will look for, etc -
https://www.vicroads.vic.gov.au/-/media/files/formsandpublications/registration/vass-signatory-contact-list---light-vehicle.ashx?la=en&hash=69686F45F193B741E72EF2B02A6E728E