well if you change the bowl youll change the swirl, for one. And that 12v head is more restrictive in order to make more swirl. So part-load operation may become laggy and smokey since the fuel isnt being completely burned.
The 24v head is less restrictive and makes less swirl and with a ppump injecting at a lower pressure you may never get it to run exactly how you want it.
And monotherms arent something Id buy for a street truck, thats some serious coin.. more like a puller mod. If youre building a puller you want airflow, lots of it. swirl probably isnt an issue.