smaller flywheel from 18GB

I posted in the Trader section for a smaller early B flywheel. Maybe I should have posted here first in order to get my ducks in a row...

here's my problem:
I'm going to install a 300ZX transmission on the back of an 18GK 5 main motor for a tube framed "period" MG street special along the lines of Miles R1.
The standard 13" flywheel is a squeeky fit inside the Nissan bellhousing. The 12" early B or MGA flywheel would solve the clearance issues, and I thought that it would also be considerably lighter, killing two birds with one part. I'm told by different folks that a)this is correct, and that b) no, the 12" flywheel is if anything, heavier than the 13".
Can anyone say what the weight of the 12H1747E flywheel from an 18GB early three synchro trans B is? The 18GK four synchro 13" wheel goes about 19# and change on my scale. I picked up a Magnette 12" wheel, and it goes over 28#, and is visibly thicker.
I found a reference online for machining down the MGA wheels, which should work for the early B wheel as well if that is what I have to do. That does start to carve up the budget, however. Have not had a flywheel lightened for about 25 years, so am sure that I'm in for future shock...

MGA, 18G, 18GB, and 18V flywheels are all different. I'm not sure you can even mount a 3-main flywheel on a 5-main crank. Early B flywheels are indeed smaller around but I THINK heavier. If I remember tonight, I will weigh the GB and 18V wheels. There's only one G flywheel here, and at this point, it is staying on the engine.

An early 3 syncro flywheel will not fit onto the later 5 main crankshaft its a different pattern and one uses studs and the other bolts, also you will have a big issue with the ring gears as one is a totally different tooth count and one is a pre engauged starter and one is not

a guy already did this 300z trans converson and its listed here on this site

The 280Z tranny conversion uses the smaller 3 synchro 5 main flywheel. I have one on my GK engine. Mine was only 16 lbs, but I don't know if it had been lightened. They were available in something like 5 different weights for different applications. John Twist had a write-up on flywheels a year or so ago.

Well, I was surprised. The 18GB flywheel weighs 20lb, 1 oz., while the 18V flywheel weighs 21 lbs, 10.5 oz.