I m planning change the stock heads in my ´87 LG4 and I need to know if L31 vortec heads fits properly in it.
what kind of intake manifold should I use?
Is another modification neccesary?

 

You'll just need a Vortec intake to match the Vortec intake pattern which is different than the LG4's. You'll also lose you're EGR function, whether or not it matters in Florida I'm not sure.

 

You'll also be putting 64cc heads on cylinders that have 58cc chambers for the 9.5:1 CR. If you put them on a 305 and don't have plans for a 350 in your future, you should shave the heads to 58cc, which is approximately .04" off each. If you do that, have the intake manifold shaved, too.

You will also need self-aligning rocker arms. If you plan a cam upgrade, the springs will have to be changed and the guides cut down. You'll still have press-in rocker studs (AKA "pull-out"), having them machined for screw-in a good idea while you're at it.

You can get a Vortec EGR spread-bore intake. You'll have to plumb exhaust gases up to the intake from the exhaust manifold/headers, that's all. If you don't make the EGR functional, you can have mid-throttle pinging problems.

A set of World S/R Torquer 305's p/n 0427050-1 is another alternative you should consider. With some minor pocket porting, they'll flow as well as Vortecs, they're high-quality aftermarket castings (with lots of room for porting), have screw-in rocker studs, come with springs that support up to .500" lift (easy to upgrade for more), and use standard intake pattern and non-SA rocker arms.

Add it all up, I think you'll find the Worlds are as good a deal as Vortecs.