Body to firewall fastners

[MonstaS4]

I'm at the stage where I need to get the body buttoned down before starting on the doors. I've planned to use bolts rather than rivets. Initially, I was planning to use plus nuts but I'm now questioning this. The plus nuts have a 2-3mm flange and if I install them now (unpainted) wouldn't this make paint prep a pain? It'd be pretty fiddely sanding around all the rivnuts.
Another option I'm considering is using U-nuts:
136440

I can easily install a few to keep everything buttoned down while roughing in the doors, then remove when prepping for paint.

What have others done here? I assume weatherproofing is another concern?

[House Money]

I'm at the same stage with my truck build, I also considered all the options and came to the exact same conclusion. I'll be interested to hear what others have done.

Gary

[FF33rod]

For the various attachment points....

Attaching to the firewall, many of us have used rivnuts for plastic, 10-32. Some went 1/4. You can order from mcmaster carr. Yes they get in the way of sanding but all you have to do in many locations is sand for paint adhesion not mirror flat. The firewall is hiding the paint. Note these rivnuts hold better than the kit ones. Nonetheless I still had one spin on me so i ended up using a dab of panel bond on each. Its also a good idea to chase the threads of the rivnut once in place

Door sills i drilled and tapped the frame, again 10-32. You likely need spacer/shim behind for support.

Any place thats visible (roof attach, cowl access covers), you can countersink them so they are flush.

Steve

[Svtfreak]

Steve speaks the truth. The rivnuts there are hidden by the firewall. Sanding there is for adhesion only. Little roughing up before you put them in and you good. After paint, run a tap through them to clean them out.

[RoadRacer]

Door sills i drilled and tapped the frame, again 10-32. You likely need spacer/shim behind for support.
Steve

What did you use for sill shims and how thick Steve?

[FF33rod]

To be honest, I haven't done that yet - I have the screws in place but not the support behind. I'm thinking either individual washers or flat stock (aluminum or steel, whatever I have left over). From what I recall, drivers side will need about 4 washers and passenger side maybe 1 or 2.

[MonstaS4]

I've decided to go with the threaded U nuts, mainly due to tolerance. I've had issues previously where the rivnut ends up slightly off centre (mostly due to operator error :P), then had to enlarge/elongate holes to get bolts to fit.

Good point regarding the sill shims Steve. I measured mine today and found a fairly even gap of ~8mm both sides.