I've done something a bit similar on the F-40.  A square box tubing frame is welded together, and perpendicular mounts welded to the frame. Then glassed to the rear bumper.  Then bolted to the ears I welded to the metal bumper support.  Front was the same except I have a single bolt monted in each attachment point so the nose can clam shell open in front.  Side pieces were glassed to metal supports welded under the door sills.  And I welded box tubing out from the front just before the door to recieve the front part of the side pieces.  Same for the rear near the opening of the hatch.  these pieces have alignment pins that come throught the glass.  (For the rear hatch and front hatches to align when they close)

I can't speak from experience but:

You should never body the body directly to the chassis without allowing for some flex. Your chassis will flex while you drive, and if the body is glassed in, how will it flex with it? Eventually you might develop spider cracks.

You can build the subframe and bolt it to the car. But as for the connection between the subframe and fiberglass, I think you have at least 2 options:

Some people have glassed in bolts (head in the glass), and then bolt it to the frame with a urathane spacer in between. Ideally you will want to put all of your bolts in compression where they are strongest, and not in shear where they are weaker. I suppose this cannot be avoided on the skirts or something like that.

Another option is to make plates on your subframe for your fiberglass to attach to. Then you can use a spray on urathane (3M Window Weld #051135-08609 or 3M #8463 foam - you need a gun for this one) to coat the plate. It will take several days to harder but it will be strong and flexible enough to support your body. Here's a guy that made motor mounts with the stuff:
http://importnut.net/motormount.htm

Some people have also suggested mounting the body in pieces, like the original car. This is a project in itself, since you need to fiberglass small lips for the panels to fit on top of each other. I would think it is the right way to go, especially in the case someone rams into you, but again, it is a lot of work.

Im sorting of doing the same thing, and Im using a method I learned from www.paradigmmfg.com. He uses a bracket welded on to his body with rubber grommets in between washers to allow for vibration and movement. He built a kit car
from scratch for under 10,000 and it looks good. He has a pamplet for 16.00 dollars that I bought, and I highly recommend because the info is great. Hope this helps!