This is taken from the 2021 NHRA rule book general regulations. It looks like the cross bars and rear bars definitely need to be welded to the main hoop. Be tough to get it in and out of the car if bolted to the plates on the floor.

4:10 ROLL BAR
All roll bars must be within 6 inches of the rear, or side, of the
driver’s head, extend in height at least 3 inches above the
driver’s helmet with driver in normal driving position or be within
1 inch of the roof/headliner in the area above the driver’s helmet,
and be at least as wide as the driver’s shoulders or within 1 inch
of the driver’s door. Roll bar must be adequately supported or
cross-braced to prevent forward or lateral collapse. Rear braces
must be of the same diameter and wall thickness as the roll bar
and intersect with the roll bar at a point not more than 5 inches
from the top of the roll bar. Crossbar and rear braces must be
welded to main hoop. Sidebar must be included on driver’s
side and must pass the driver at a point midway between the
shoulder and elbow. Swing-out sidebar permitted. All roll bars
must have in their construction a cross bar for seat bracing
and as the shoulder harness attachment point; cross bar must
be installed no more than 4 inches below, and not above, the
driver’s shoulders or to side bar. All vehicles with OEM frame
must have roll bar welded or bolted to frame; installation of
frame connectors on unibody cars does not constitute a frame;
therefore it is not necessary to have the roll bar attached to the
frame. Unibody cars with stock floor and firewall (wheeltubs
permitted) may attach roll bar with 6-inch x 6-inch x .125-inch
steel plates on top and bottom of floor bolted together with
at least four 3/8-inch bolts and nuts, or weld main hoop to
rocker sill area with .125-inch reinforcing plates, with plates
welded completely. Also, the roll bar may be welded to frame
connectors that are fully welded in place and are 1 5/8 inches
x .118-inch MS or .083-inch CM round and/or 2 inch x 2 inch
x .058 MS or CM rectangular. All 4130 chromoly tube welding
must be done by approved TIG heliarc process; mild steel
welding must be done by approved MIG wire feed or approved
TIG heliarc process. Welding must be free of slag and porosity.
6” max
All cars with an OEM frame must
have roll bar attached to frame.
Cars without frame use 6” square
1/8” steel plates on top and
bottom of floor, securely bolted
together with at least four 3/8”
bolts, or top plate welded to
rocker sill.
All materials must be 1 3/4” O.D. x .118” mild steel or .083”
4130 chromoly tubing, except A, which is
1 1/4” O.D. x .118” mild steel or .065” 4130 chromoly tubing.
5” max
4:10
A
Maximum of 6”
from roll bar to
driver’s helmet.
Maximum of 5”
from top of roll
bar to brace.
Section 21, page 22 General Regulations
Any grinding of welds prohibited. See illustration. Roll bar must
be padded anywhere driver’s helmet may contact it while in
driving position. Adequate padding must have minimum 1/4-
inch compression or meet SFI Spec 45.1. All cars running 9.99
(*6.39) or quicker, SFI Spec 45.1 mandatory.