That Isuzu bell is stout (never noticed how thick those flanges were!), but is also pretty difficult to find... IIRC Marlin ordered the ones for Michael and Corbin's trucks straight from the dealer. I'm surprised that the friction shops can't help?

The flywheel space/adapter seems like the cheapest option, if you can really get one for $135 (I'd have guessed $300, myself - and I'd be fine with aluminum in this compression application). I'd separate the pilot bushing and spacer for less cost and complexity.

The spacer could be as simple as a flat donut to fit around the little stick-out of the crank bore, with the holes drilled... you'll need less material to do a flat, mostly 2-dimensional donut than the more 3-dimensioned setup you show. It should also save you on machine time for hogging through waste, and less operations should be cheaper, as well. Machine list could be as simple as:
1.) start with 1/2" aluminum flat plate, chucked up on lathe
2.) turn OD round to same dimension as crank OD
3.) turn ID to knock out a center hole a bit larger than the pilot bushing
4.) turn a larger ID about half-deep to fit the OD of crank shoulder stickout
5.) (optional) light cut to face the workpiece
6.) (optional) cut a slight reference groove where the crank bolt hole centers are located
7.) flip the workpiece, chuck it back in
8.) face the piece down, but leave a short shoulder OD for the flywheel to engage
The bolt holes themselves could go in with a mill for perfect balance, or could be dropped in at your end with a drill press, using the flywheel as a template. Drill 'em just a hare oversize to permit radial hole misalignment -- the shoulders on the crank and adapter are what lines things up, the bolts just clamp it all together. You'll need to factor in the cost of longer flywheel bolts (you want quality fasteners here), which may be a bit of a pain to source. You could start with a thicker plate, it just depends on what stock they have and how much material you want to play to remove... I think half-inch plate would be good enough, even narrowed a bit when you provide for the shoulder.

Starter line-up will be easy if you find some negative shims. <img src="/forums/images/graemlins/pfft.gif" alt="" />

Randii (who still likes the custom friction plate)