Compressing this into one page and three pictures might make it seem easier than it actually was, but here goes . .

. . although, as we said on the previous page, the front crossmember was quite easy to fit. First, three square tubes were tack-welded across the chassis jig in positions corresponding to specific points on the chassis (e.g. firewall, rear taper start, etc.). The centres were marked, and from there the two rails were clamped into position (when making the rails, we centre-punched them at these reference points so they wouldn't be lost during welding and dressing). Because the bottom wasn't flat, the sloping front sections were clamped to uprights designed for mounting axles.
When we were happy with everything, and all dimensions, including diagonals, measured up to our drawing, we tack-welded angle brackets onto the three cross tubes on the outside of the chassis. This allowed us to unclamp one of the rails so we could get the tabs on the front crossmember between the frame horns, then clamp the rail back in place. Another check of the dimensions, and we could weld in the front crossmember, TIG-welding as with most of the other chassis welds. We didn't fully weld this part yet, as we will be partially boxing in the underside later.

We also welded in two centre crossmembers, made from 2" x 1" box section. One was fitted flush with the bottom of the chassis, the front edge at the point where the chassis kicks out for the rear half. This is just behind where the gearbox ends, and the gearbox/front wishbone (yes, we're thinking of using an unsplit wishbone for front axle location) mounting section will project forward forward from this tube. The upper crossmember sits flush with the top of the rails, the front edge level with the rear of the lower one. The offset is primarily to make it easier to fully weld both tubes, but also allows a little extra clearance for threading through things like the exhaust system. The top one will have to have a section removed from the centre later on, because it is at the same height as the propshaft (a penalty for making the car so low is the relative height of the engine/gearbox/propshaft through the cabin).

Once the front half was ready, we could get on with mounting the rear subframe. Another check of the dimensions, and we could tack the frame on top of the rear supports - the careful checking during preparation of the rails meant that it fitted so well, we only had to bevel the edges before welding. When securely tacked, we had to flip the chassis over to finish the welds as they were all on the underside of the subframe.

Perhaps it was that easy, after all? We didn't seem to have any trouble making bits fit, and even after all of the welding, the chassis dropped back into the jig with hardly any shrinkage or distortion, and it all seems to be square.
The rear of the subframe drops away slightly, because of shrinkage from the welds on the underside, but we should be able to counter this when we fit the crossmember for the rear suspension.