BoneDaddy

Yeah its oc and can reach 279w at Furmark acording to techpowerup, less during regular use tho. Guess it’ll be fine and im gonna go for 6->8pin for the science (and the part of me that hates the bus and is super lazy). Thanks for the help, appreciate it:)

PSU is plenty strong enough.  65w CPU, 5700xt 225w (and you are right, 6 + 8 pin is what they should come in btw - maybe yours is a factory OC?) and 15-20w for ODD,HDD,SSD, fans.
 
Heres the thing - you can try anything I.E. slot gives 75w, 8 pin gives 150 (that's the 225) then hope that the other 8 pin slot doesn't draw more than 75w and your 6 pin to 8 pin will work just fine.
 
BUT I have the same option as you (free 6 pin), and knowing what I know I went with 2x Molex to 8 pin (bought 2, only needed 1, got the second cause I may need one in the future and it was super cheap shipped).  Im powering an R9 Fury Nitro+ so couldn't take the risk with a 6 pin to 8 pin with that power hungry manimal.

I would get 2x Molex to 8 pin.
 
6 pin delivers up to 75w
8 pin delivers up to 150w (double, may melt the adapter if it puts that current through or just crash)
4 pin molex delivers up to 132w on the 12v pins (from the way I understand it)
 
And since you are adding restriction in the current by using an adapter that's why I suggest getting a 2x molex to 8 pin so it shares the burden (and doesn't melt under load ) 
 
Just an FYI I just bought 2 of these 2x molex to 8 pin for a rig for these exact reasons described above.
 
 
However there are much more intelligently versed people here so wait till they also chime in

This will work fine, there are dual sata to 8 pin adapters, but I would advise against those because most of them have the crappy molded plastic SATA connectors that created the saying "molex to sata loose all your data," and I couldn't find any adapters on amazon that had the better crimped sata power connectors.