How would a gear change from 1:32 to 1:47 afect the performance of my boat?

You'll see more mid range snap and a bit more top speed

Thanks, how do you think 1:47's will hold up behind my big blocks?

What's the boat and how much hp?

32 Pachanga with 330, 454's, Im not to hard on them no hard launching and not to much air time

If you do plan the 1.32 to 1.50 swap then spend the extra cash for the Merc HD upper 1.50 gear set. They are stronger than the regular 1.50 gears. GLM also makes the HD sets. I've ran both the Merc and GLM HD gear sets in my Alphas behind a 400hp 355 and 450hp 385ci SBC.

I contacted GLM and they told me the 1.50 gearset was the HD's ,just wasnt sure if they would hold up thanks for all the input

Ryan,
Is there any difference in the durability of the GLM vs Merc gears?

I really can't say. I've ran both and have had no problems. The GLM's look just like the Mercs appearance wise. I ran the GLM's last summer behind a 400hp 355ci for 136hrs and spun it 5700rpms @ 72-76GPS. I found a set of Merc HD gears and built another Alpha with them for my new 385ci 450hp engine.

what prop are you running now, you will have to go to a larger prop to keep from overreving, larger prop means more stress on outdrive and will possibly loss some out of the hole performance. check out this web site and play with some numbers.
http://www.rbbi.com/folders/prop/propcalc.htm