The Alfa Spider engine and gearbox assembly uses a normal three-point mounting system: a rubber 'biscuit' each side of the engine, and quite a soft rubber isolation mount - a 'Silentbloc' - at the very back of the gearbox. This mount is not solid rubber, and allows quite a bit of movement at this end of the unit.

The engine is actually tilted to the left by about 5 degrees in Spiders and Guilias, but we're going to mount ours vertically (for looks), and the engine mounts will be made accordingly. This means that the gearbox will be rotated to the right a few degrees. We have made our mount horizontal, and will rely on the flex of the Silentbloc to compensate: it looks like it should be OK, but if it doesn't last we'll just make a new angled bush.
If you look closely, the 'ears' on the mount in the top right view are different to the ones below: this is because we later had to move the engine forward for additional clearance (we'll explain why elsewhere). The grooves in the top one were to allow the bolt to slide through, but we welded the new brackets 1/8" lower on the 'Mark 2'.

This is the lower centre crossmember assembly, that provides mounting points for the gearbox and the front wishbone. There is not much to it, just welded up from a few pieces of 50 x 25 x 3mm wall box section. There are four threaded bushes welded in for the gearbox mount - the flanges on the mount rest on the top of the crossmember, so the bolts just stop it moving around, and take no weight.
The wishbone mount is a row of four brackets: this provides for either one or a pair of 3/4" spherical rod-ends. Why the choice? To give us the option, as at the time we hadn't decided whether to make a true wishbone, or to split it. We also added a couple of gusset plates, made from the same 1/4" bright steel plate.

The front tubes were cut to fit between the chassis rails by the front kick-up joint, then the assembly was welded in place. As you can see here, nothing is lost by not being able to drop the gearbox mount out from underneath, as the wishbone mount prevents the gearbox from leaving in this direction anyway (by the way, the photos show the first iteration of gearbox mount). This shouldn't be a major disadvantage, as the engine and gearbox unit can be pulled, easily, straight out the front (after removing the rad, of course). It may have been a problem if we were using an automatic transmission, as the sump might have fouled, but our manual 'box clears it comfortably.
If we've got our history right, one of the reasons for early hot rodders splitting their wishbones was to allow different engine and transmission combinations to be more easily swapped in and out of A's and B's. We are going 'unsplit', or at least still triangulated, because we're using a round tube axle, and these don't work properly with split 'bones or radius rods.