Some of the late Sixties annual kits had those stub axles, which were probably aluminum rivets of some sort. They didn't work too well because they left the front wheels on the wobbly side, and unless you got them in really good the front wheels ended up bowed in at the top. I think AMT was trying to eliminate axle holes in engine blocks at the time, and that was an attempt at a solution. Then for 1970 they went in the other direction, and put those huge diameter axles in some of the kits...
They are capable of making as good a product as anyone else. The sticking point is, you've got someone else telling them "we want X number of these, we want them now, and we're willing to pay X amount per item". Good, fast, cheap...pick any two.
The Revell kit is 1/32 scale. Monogram (long before merging with Revell) issued '55-'56 Cadillac kits of their own. For both years they offered convertibles and hardtops (hardtop kits used the convertible body with a separate, add-on roof). The Monogram kits were 1/20 scale. AMT may have made a promotional model in 1/25 scale; if so, it would have been molded in acetate plastic with a stamped metal chassis and (probably) cast metal bumpers.
Fill the groove from underneath. You can cement strip styrene into the groove and then trim it flush after the cement sets, or use epoxy putty. After you are done with the underside, block sand the hood up top.
I haven't done one with the primer/body color edges yet; the rattle can overspray sometimes looks out of scale (to me, anyway). If I were to try doing a chassis that way, I'd probably use a template or mask spaced away from the chassis a bit, to make the overspray line a bit "fuzzy" while keeping it to a minimum. Two masks might be needed for each side...the outermost couple of inches of the floorpan probably wouldn't have caught any overspray because of the rocker panels, but I don't think that the overspray would reach too far in on the underside of the floorpan either. I usually go for the "undercoating" look, because it darkens the underside of the car and (to me) doesn't draw attention away from the exterior of the car when it is sitting on the shelf. When you turn it over, though, there is at least something to look at. I paint the exhaust system first (used to use Testor's silver with Dullcote over that; on recent jobs I've used Humbrol silver which dries hard without needing anything applied over it). I'll paint the molded-in fuel tank a darker metallic gray than the exhaust detail, but not as dark as the cast iron parts. I then paint the adjacent edges with a fine brush, using an acrylic like Polly Scale Steam Power Black (one of the "blackest" black paints I've seen; not dead flat but not really a semi-gloss either). Polly Scale isn't being made anymore; I laid in a supply of a few bottles and am keeping an eye open for an equivalent to use in the future. Don't drag the brush back over work already done; doing that is what usually creates brush marks. I'll do the areas that look the most difficult first (like where the exhaust goes over the rear axle) leaving the nice, straight, out-in-the-open, easy-to-get-to areas for last. I clean up the edges as I go, using a pointed toothpick to rub off any black acrylic that strays onto the exhaust or fuel tank. Every so often, I'll clean the brush and re-mix the paint so everything is consistent. I let that dry, then use a bigger brush to get all of the other black areas. If I need to, I'll apply a second (thinned) coat of the black later to get rid of any brush marks or other inconsistencies. If there is leaf spring detail on the rear suspension, and it's well-defined, I'll pick that out with some steel gray (darker than the fuel tank but not as dark as cast iron gray). I'll only paint the very "top" of the leaf spring detail. I don't try to freehand any part of the chassis that "rises up" to the spring detail from the sides. Sometimes the rear shock absorber detail is clean enough to pick out...if so, that can be done with some odd color (with those parts, color varies by brand) when the exhaust and fuel tank are done. If you decide you don't like the way the shocks stand out, you can go over them with the black later while doing the rest of the chassis. I don't do anything with the molded-in rear axle...to me, that's probably the worst area of this type of chassis, so painting that with the "undercoating black" makes it fade into the background, because it's not something that you want to stand out in any way.
The bottom piece is a sanding stick, with different contours to get into different areas. In the '61 Styline kit, you got a little tube of putty to mold in the extension pieces, and also a sheet or two of sandpaper. You could glue the sandpaper to the stick.
The one store I went into every so often only had six or eight different kits, nothing special. Lately, every time I go into that store, I walk out wondering why I went there...that's probably why the visits have tapered off considerably. The Michaels craft store chain appears to have reset their model kit section...smaller than before, most Round 2 kits gone, most Revell glue kits also gone, replaced in large part by Revell snap kits. They're still good for paint, embossing powder, craft wire, airbrush jars, and all sorts of other stuff though.
I saw two of them yesterday at a local shop. They charge full retail, though, so neither of those followed me home. I'll pick one up at another local shop that at least gives club members 10% off (offsets the sales tax, at least) then pick another one up online or at a show.