here are some tips for building a cage.
1. Make sure the material you are using is sufficient in strength for the weight of the car.
1.75" .120 wall DOM tubing is a standard for something in the 3500-4300 lb mark.
1 5/8" is not quite enough but with alot of triangulation it can be made to work (heavier in the end)
2. The ends of your joints need to be tight to each other, DO NOT just get a mig and weld some tube together. You NEED proper tools for the cage,
Fishmouthing the ends of the tube is pretty straight forward, you can use a chop saw to make 45* cuts to create a fishmouth.
If you plan on having a car for a while, make the cage part of the chassis. it will strengthen it up. Plates to the floor are maditory where the cage bolts to the floor.
When you have the height of the cage and you need to weld to the top of the cage but cannot take it out..... Drill holes in the floor that will allow the cage rails to drop through the floor, allowing access to the top of the cage, then after finished welding, place some dynamat underthe plate and use that to seal the floor hole back up under the base plate.
Making a cage is not just a " I have a welder and can tack tack tack"
There is a reason the prices for internal cages for cars is high... there is ALOT of work to go into makeing and desiging the right cage for the car as well as keeping it looking good.
If what I wrote here is confusing. Buy some offroad magazines, read and ask questions before you just go out and build a cage.
As I say to some people; do it your way, screw it up and then get it done the right way so others can learn from your mistakes.
Good luck.