How much crank end play would you want with an alloy block at room temp 20*C. I have a new crank & using King Main bearings I have 0.003 clearance. Crank spins nice with mains torqued up but is this enough end play for aluminium?

The manual I have says the factory spec was 0.002" - 0.007", with 0.010" maximum.
My aluminum block has 0.005" with no issues.
Good question on yours, but I don't have experience that tight with an aluminum block. But my feeling is you are OK, let's see if anyone posts experience.

I have always ran .007 - .0010 . What block is it ? My Indy block the crank was hitting one of the caps and it needed a little touch up on the mill to allow the crank to float as it should . Check all the caps for clearance also

I agree with checking to make sure the crank isn't touching. Also I take a rubber mallet and seat everything both front and rear of crankshaft. Aluminum block is going to grow so you should be good. Birdtracker

try loosening up the center main cap bolts, all of them, and then try whacking both ends of the crank with a big soft mallet and see if that will help centers that cap and give you a tiny bit more clearance
I've had to do that more than once on many different motors
I shoot for .0040 to .010 end play using a dial indicator on the snout end of the cranks using a large handle screwdriver to pry the counterweights back and forth
Get it correct and go forward with having fun without worries on it

It is an Indy Maxx block, with a Molnar crank. I did seat everything with a rubber mallet as we torqued up the main caps as was suggested & it does spin really nice, but I just wasn't sure whether 0.003 was enough, but someone suggested that being aluminum, it might grow when hot. I was just hoping someone could confirm I was good to go.

You might want to try adding a little more clearances, .0015 to .0020 on both the front and back sides of the thrust bearing by sanding it on some medium grit sandpaper on a really flat surface, measure carefully while doing that
Drag race motor live a lot longer with looser rotating clearances than they do with them a tiny bit to tight

I agree with Cab…..and if you do this, place the bearings together (same orientation as when installed) when clearancing them on flat surface , to keep them square. As opposed to sanding each half separately.

Forgive me if I'm mistaken, but aren't you supposed to take it off the front side? I agree with an alloy block needing another thou or 2 because of the aluminum growth.

.003" seems tight to me but I've not done an aluminium block yet.

We have ran them as tight as .006 and as loose as .010 with no issues. Aluminum block 540