The external pump is an idea. Wouldn't take much for that.Well the tranny should take a while to pump up after it's stopped but Mizlplix eventually ran a external pump and a T&C flat track converterless kit. If it was a powerglide you'll wear the low bands out, I won't claim to be qualified on anything modern.
I run an alternator, back when I converted, dc-dc boxes were too low power and unreliable and I'm too lazy to redo, besides that's where the ac gets driven from. Figure 2hp at idle about the same as the power steering so 3-5hp don't sound all that wasteful even if it's 3kw.
Only thing I can think of is run it like a chevy small block and keep it in gear until 4,000 then shift up if you have the battery headroom to go that high. That should put you at 2500 max power but cooler. I have a 5 speed, so any time I'm below 2500, it goes down a gear even on the freeway. Kostov told me they tested them to 6000, mine has been to 5500 once (oops)