i have a 4.5 inch spring in the front with a 3 inch spacer. My question is can i use a 5 inch block in the rear with a 3 inch shackle?

 

If i were you i wouldnt use blocks at all. I have never used them but i hear nothing but bad things about them. just my two cents.

 

i have 2in blocks in the back on mine and it rides like dog crap i hate them! i didnt even know they made 5in blocks. Your leaf springs will probably also snap and break after awhile of being inverted. I wouldnt do it.

 

This'll be good.

 

ive got my popcorn

 

The answer to your question is YES, you can. The follow up to that is, PLEASE POST PICS of your destroyed rear end when you spit those blocks out and bust everything under the sun.

EDIT; in all seriousness it's a very bad idea. Since this is your first post, I'll go a little further and ask why do you need that much lift? What size tire are you running? What axles are you running? Do you wheel it at all, or is it a street queen? Give some more details on your setup and how you want to use it.