I went through the same decision process last year for a 40A Renogy DC to DC for a 200 Amp-hr lithium system. I asked questions here and searched the web to determine a path forward. Scalf77 (greg) suggested upgrading the alternator and I also thought that was "overkill" but later changed my mind.
I went on to try and design an adaptive controller to switch the DC-DC between 40-20 and Off adaptively based on Sensed alternator temperature using an Arduino with the display.
The basic issue is that at 40 amps while being a reasonable load at RPM/speed, is too much for a standard alternator at idle. This is the reason in a nutshell (similar to Greg's numbers).
If the DC to DC is outputting 40Amps at idle (12V at the battery and 14 amps to the Lithium) then it needs to input about 40*(14/12)*1.2=56A into DC-DC. The 1.2 is a power conversion efficiency number and the 14/12 is the current increase due to the DC-DC voltage boost (12V in 14v out).
Still worse is the fact that depending upon the alternator's efficiency (Victron claims 50%) then you would need to be generating 50% of your output power in excess heat. If we just assume 80% efficiency then this is equivalent to 67 amps. So idling is a worst-case situation that will overheat the stock alternator that otherwise at highway RPMs would be fine. If you lose a volt in cable voltage drop from DC to DC from the Alternator then you are at 40*14/(12-1)*1.2*1.2=73 amps (Equiv )
So while I was dismissive of the "larger Alternator" idea, when I saw the enhanced electronic controls of this alternator at a very reasonable price I jumped on it and feel comfortable that the alternator thermal limits will keep me from getting into trouble even if I forget about manually controlling the DC as stated above.
"This Is Truly A High Amp Premium Quality Well Built Alternator
It Has OE compliant functions : current limiting, thermal shutdown,
load dump protection, shorted field & lamp circuit protection"