Engineering problem with my 1984 Volkswagen

[MotorradMike]

Hi guys:
This time, it's the alternator belt pulley set.
I had to change the water pump and housing and got OEM parts.
There has always been a slight alignment issue with the 3 pulleys not being co-planar and the car has been hard on belts.
Now the new water pump sticks out about .140" further than the crank pulley and these 2 almost touch so not enough distance for the belt to deal with the misalignment.

I've decided to make a spacer to make the crank pulley stick out to match and I'll probably space out the alternator as well although it's far enough away that the belt might deal with it.

Here's my Engineering question:
Can I make the spacer from Aluminum or does it have to be steel?
I'm just wondering about expansion/contraction letting the bolts get loose. I could Loctite them but I'd rather not, they can be a bitch to get off as it is even though only torqued to 15 Ft-Lbs.
Maybe you will say I'm overthinking this but I had a crank pulley come off once on a different car.