I think I'm missing a nut.

the crank snout is probably full of 50 year old road dirt and rust

id fill it with wd 40 or penetrant oil and leave it for a day or two

then blast it out with air or hose it out
and try again

a tap might be a good idea

the puller/installer i list would get the balancer off. it has a snub nosed fitment for the main pipe that can sit on the end of the crank without damaging threads and a thin one that can fit to the base of the hole if need be

but you need the thread for either method to get a balancer back on

unless you have the motor out
the facilties to stand it on its crank flange in a press
and press on the new balancer

whacking it on with a mallet is going to be a long and awkward process
its not going to be tapping.

in engineering terms i'm not sure what "fit" they used
but its not a slip fit

Plumbers freeze spray on end of crank
graphite powder
and the balancer warmed to say 60-80*C
might help

elastomeric rubber is good to about 130*C but can't say for the bonding agent between it and the balancer parts . so i kept it to a temp that conceivably a balancer might reach on a hot day in a hot car