I definitely agree with cutting holes in the floor for the 4 main vertical bars so you can drop it down and fully weld the top corners. When you get those done and push it back up, get it as high and tight to the car as possible, even if it means building plinth boxes on top of your spreader plates. A lot of things can be redone later if necessary, but maximizing head room and interior space you can only get right once.
RCC kits are pretty good, but they do require a little fitting. First, you'll need to notch the halo to the main hoop. More of a pain in the ass is notching the front uprights (A-pillar bars) to the halo. Try to have these hit the halo as far forward as possible to keep the bars tight up against the car's A-pillars. A lot of first-time teams (my team included) make the mistake of landing the front uprights too far back on the halo. This weakens the cage, restricts entry space into the car, and makes notching the bar where it meets the halo harder due to the shallower intercept angle.
You'll also need to cut all 4 vertical bars (front uprights and main hoop) to length wear they hit your floor. Safest to leave them a little long; cutting more off later is easier than building a plinth box to raise the floor. You'll need to do the same with the backstays.
It's generally a good idea to only tack things together until as late as possible in the process, so you can make adjustments if you have to. Even so, be very careful grinding off tack welds. Better to leave a bit of extraneous bead on the tube than nick the tube's OD. Lemons tech doesn't like that.
Finally, it's best to have your seat placement finalized before you start on the cage. Your seat back location might affect the placement of the main hoop.
Command Pilot/Flight Director, Escape Velocity Racing
Index of Effluency, Heroic Fix, Class C Trophy, now hopelessly gunning for Class B.
Currently 1-2 vs. Team Fairlylame in the Class of 1964 Championships