I'm a little late to the thread with this, but I have some info that you guys might find useful. The L21 gets cam position the same way an L29 does - from the distributor's 1x sensor. The distributor assembly in an L21 is exactly the same as the L29, still driving the oil pump, except that it has no ignition cap/rotor installed, just a blanking cover instead. This assembly has to be installed and clocked to correct timing position exactly as a full L29 distributor does. The L21 has a specific 24x tone wheel in place of the L29's 4x tone wheel, and a specific pickup.
All L21s came with drive-by-wire throttle bodies. There is a better alternative than running the antiquated PCM they came with. Use a 411 and wire it up exactly like an LS. The component that makes this possible is a Vortec 8100 camshaft. The 8100 has the LS firing order and makes it unnecessary to to reassign the firing order banks in the PCM. You can start with a 6.0L/4L80-E base calibration and adjust for displacement and VE, and you've got yourself a combination that anyone with LS experience can tune.
I've got a ZZ502 sitting on a stand. It's got Brodix Race-Rite oval-port heads, a ported L29 long-runner intake, blanked L29 distributor, L21 rocker covers and LS1 coil packs, LS1 coil pack harnesses, L21 24x tone wheel and crank position sensor, and a Comp Cams TPX262HR-14 for a Vortec 8100. I haven't fired this combination yet because I haven't built the harness yet. The harness I'm starting with is a VIN J 454 GMT400 Suburban, it will be pinned to 411 connectors, and have the coil pack wires added. Should work great and run everything just like the stock PCM. (motor and PCM going into the only version of a 2000 C1500 GMT420 ever made - aka Tahoe Limited.)
Incidentally, that 58x timing set from GMPP was originally designed by EFI Connection as hybrid between the Gen-VI (454/L29/L21) lower half of the cover and the Gen-VII (8.1/L18) upper half of the cover in conjunction with the 8100's cam gear and cam sensor, L29/L21 crank sprocket, and the L21's tone wheel and pickup sensor. They eventually came up with a 58x version. GMPP now makes and sells that same 58x setup because they needed it for the ZZ632, with the only difference I can tell being that EFI Connection's setup used a stock single row timing chain and the GMPP version can fit a double-row due to the additional area underneath afforded by the bump-out in the tone wheel area. This also means the balancer hub on the GMPP version will need to be shorter since the crank sprocket is the same thickness a standard BBC rather then the thinner L21/L29 which is thinner by exactly the thickness of the tone ring. There's plenty of clearance between a Gen-V/VI balancer and the aluminum timing cover (1-piece rear main seal) for them to add some space for the thicker double-row setup, so shortening the balancer hub for your combination won't pose a problem.
