I found an iron block ls1 to go in my truck it has 706 heads on it, what kind of compression ratio should it have?

 

There's no such thing as an Iron block LS1. It sounds like you've found a 5.3. Can you verify if it's been bored out to LS1 specs by chance (3.91)?

 

Yes I can verify the motor is a 5.3 bored to an ls1 an has 706 heads installed on it. The motor runs, but I wanna know if I can put it in my 05 gmc sierra and still run pump gas

 

K... that's what I wanted to know.

The 706 heads have a 61.15cc Chamber

The 241 (stock LS1 heads) have a 66.67cc Chamber

So it'd be like running on a set of .035 milled heads roughly. (using .006 = 1cc)

Roughly, you're looking at 11.1 : 1 CR according to LS1tech:
(https://ls1tech.com/forums/advanced-...ume-vs-cr.html)

I would say as long as you're planning on running 91+ oct. with a tune that's spot on, you could get away with it, I would wait for a couple other guys to chime in just to verify though, I don't wanna be the a$$hole responsible if you ruin the motor.