I replaced my servos with Hitech also except I went with 645mg all the way... I used the ofna purple servo horn.. it is a little loose on the ends as you said.. I used a homemade adaptor rigged from du-bro airplane hardware... I chose to leave the stock horns that came with the truck on the steering for 2 reasons... 1.. They fit with no modification......
2. I can't see them, so it's not important how they look...

part of the fun is being creative with the trucks.. experiment with linkages and see what works for you.. just be careful of the cylinder head... I tried to mount the linkage on the bottom,but it didn't work well...