The 6300 DC fuse panel has two pos 12v wires from the converter. Red and blue. White to the neg set-screw terminal. The newer converter will have one pos (blue) and one white.
So to use the 6300 fuse panel with the one pos wire from the new converter, jumper the panel's two pos terminals and put the one blue wire to one of them. Now the new converter powers both sides of the 6300 panel.
For the converter input 120v, it will likely have a plug in to a receptacle. You can do that if there is one handy. You can cut off the plug and put the hot to the breaker same as the 6300 was and the neutral to the neutral bus same as the 6300 was.
BUT--you want a switch for this converter so you can turn it off when on inverter. If it is a plug in just use a power bar that has its own on off switch. If you do a hard wire, insert a switch on the black hot wire before it goes to the breaker. I did that with my 7300 in a previous RV, using a stick house light switch for the converter switch. Remember to test it before putting it in so up is on and down is off.