I was also told to replace the trem block in my 98 MIM Strat with a Fender 62 reissue (MIM is zinc), or the Callaham. Will this upgrade make a big tone change?

Is your current block actually zinc? Fender was using a mix of some powdered metal and epoxy for quite a while. That's what my MIA 1996 had. I upgraded to a steel block and the difference was substantial. Close to the change you get when you switch to premium pickups.

Go with steel or stainless steel saddles, and a steel sustain block. Fender has used a lot of zinc saddles....replace 'em.

I found though, that the Wilkinson trem is excellent and has a zinc block. The fact that it has big stainless steel saddles might make a difference.

I had a trem from Guitar Fetish. Inexpensive and it had a steel block. Sounded great. Really made the strings ring. Must have had bad saddles, though, because I couldn't go more than a day without snapping a sting. That was getting expensive. I switched to graph tech saddles on that bridge and that killed the tone...I hate those things.

I currently have an 08 MIA with the new all metal sustain block and that sounds acceptable.

It's really not a subtle thing when you have a good sustain block. The strings (you really notice it on the lower strings) really ring out and sound more focused. You notice it plugged or un-plugged. A good block is a must-do on a Strat...it's a shame there are so many Strats out there with the crappy blocks and the players don't know what they're missing.

Do it.

I replaced my trem block with a callaham and it made a huge difference -
more sustain, more chime.

I'm also in the "go for the Callaham" crowd. I got one in my '96 MIM and it made a HUGE difference...as Siamese said, nearly as much so as putting in good pickups. The first thing you'll likely notice is considerably more sustain...on this alone a good steel block is worth it. As far as tone goes, I wouldn't say that the block so much "changed" my tone as much as there was simply more of it. About the best analogy I can give you is like listening to your favorite song thru a small $10 mono FM transister radio and then listening to it again on a -good- stereo system...it's still the same song, with the exact same frequencies and such but you simply hear more of it.

Also, between the Callaham and the Fender reissue, I'd seriously go with the Callaham myself. I'm sure those reissue blocks are "good" and even the brass block I got from GFS for my '03 MIM is decent but those Callaham blocks just really make a world of difference. For the $60 it's the best thing you can do for an MIM next to new pickups .

Just my $.02 worth,
Jim