Having built a few Vegas and Monzas into Bonneville racers, pro street and drag racing altered cars over the last 30 od years, I will say a few things here... the cage package you are referring to is a pretty decent package- perhaps not exactly how I would do it but not far off either.
From Competition Engineering's website though:
Designed to provide the utmost in chassis strength and superior 360ยบ driver protection with proper installation
Design ties the front frame to the roll cage making it one solid unit, eliminating chassis flex and decreasing vehicle reaction time
Hoop Kit includes one main hoop, one top hoop, two front downstruts, one main hoop crossbrace and two forward struts
Strut Kit consists of two door struts, two rear struts, two diagonal hoop supports, one rear X-brace, six 6" x 6" x 1/8" steel mounting pads and twelve weld joint gussets
Manufactured from 1-5/8" O.D. x .134" wall mild steel
Pre-notched tube ends provide for a better installation
Meets NHRA and IHRA requirements
All tubes mandrel formed in-house on CNC equipment
Complete instructions are included to ensure a hassle-free installation
Welding required
I do not like the fact that their website says it is 1 5/8"....but.... As Dan pointed out, the cross member(s) under the seat are part of protection from the bottom. My personal opinion is that, since you are using a stock floor pan in the area under your feet, pay attention to what is retaining your feet in the event of the unthinkable.... and tie in the rocker panel area, the seat support stiffener and the floor itself (under your feet) with an additional crossmember... you can, in the area of the transmission tunnel, go at an angle upward and tie it into a cross support under the dash area... or go at a lesser angle over toward an area on your cage on the right hand side of the car mear, say. the top door hinge. Remember that you can also use sandwich plates with bolts to make additional tie in points to the firewall or wherever. You will also want to X brace the door areas... mostly for added strength and as a place to affix retention devices for your feet. The upper front od the added in member to make the X does NOT have to be extremely high on the door post vertical to allow relatively easy entry and exit but it should also be braced towrds a frame/chassis point ahead of the door post vertical as well.....
In our Vega, which is a 255 mph + doorslammer in altered class, several years ago, we built what I can only call a cage within a cage.... a roadster style of cage within a coupe style of cage.... and we added about 100 feet of tubing with all of the extra hoops and diagonals etc. If you want some further information, message me and I will do what I can to sketch some likenesses of it..... I am 3500 miles from home this month overseeing a rebuild of a grinding mill in an iron ore plant in Eastern Quebec, Canada (and I will be here until at least the first week of April), so I cannot take pictures and I have no pictures of the cage on my laptop computer.
BTW, the picture is the typical view of the cage from their website.
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