We re-created the 6.29-inch...
We re-created the 6.29-inch distance between the transmission face and the bellhousing flange to illustrate where the input shaft sits in relation to the Gen III/IV crankshaft. Note the input shaft falls short of the stock pilot bushing location.
Not-So Shortcuts
Car crafters are the masters of adapting parts to achieve a solution. As you might have guessed, there are other ways to arrive at adapting a Gen I four- or five-speed manual trans behind a Gen III/IV engine. One idea involves using a Gen III/IV flywheel. Bolting on a stock metric Gen III/IV flywheel is easy, but again, it will also be short by 0.400 inch. According to McLeod, a production Gen III/IV pressure plate and clutch assembly stands roughly 1 inch taller than a Gen I diaphragm pressure plate. Even accounting for the shorter Gen III/IV flywheel, this places the metric pressure plate fingers roughly 0.600 inch taller than a Gen I system. Since this does not allow sufficient room for the release arm bearing, it won't work.
However, if you have a good Gen III/IV flywheel, you could have a machine shop drill it for the standard diaphragm counterbored bolt pattern using the correct 3/8-inch shouldered bolts. As you will discover, the stock metric pressure plate bolt pattern is the same as the Gen I 11-inch diaphragm pressure plate pattern. Do not use the Gen III metric bolt holes to attach an old-style pressure plate because these bolt holes are not counterbored and will not properly locate the pressure plate. All Gen III/IV metric pressure plates use a pair of small drive pins as locators. Since this conversion will use the 3/8x16 shouldered bolts, machining a separate bolt pattern and counterboring the bolt holes for the shouldered bolts will be necessary.
This is McLeod's street diaphragm...
This is McLeod's street diaphragm pressure plate and clutch assembly designed for a late Muncie/Super T10 26-spline input shaft. The McLeod flywheel is drilled for two pressure plate bolt patterns. One pattern is for an 11-inch clutch assembly using shouldered 3/8x16 coarse thread bolts, while the second is for a metric 11-inch pressure plate using 10mm metric bolts. Be sure to match the older-style pressure plate with the traditional 3/8-inch bolt pattern.
This combination of a short flywheel and standard-height pressure plate package will also place the pressure plate fingers roughly 0.400 inch farther forward, which screws up the linkage geometry. There are actually two quick fixes for this situation. McLeod makes an adjustable-height throwout bearing that is taller and will compensate for the shorter flywheel. It runs a little less than $95, compared with the standard bearing of around $65. The second route is a taller GM release arm ball stud or an adjustable ball stud from McLeod that allows you to set the proper height of the release arm and optimize the release arm geometry. Ideally, you want to position the outboard portion of the release arm (the part actuated by the clutch linkage) angled slightly toward the front of the car. This position minimizes linkage movement while maximizing throwout bearing travel. The best choice would probably be the taller GM ball stud at less than $15.
Unfortunately, there's a downside to this approach. Because the Gen III/IV flywheel is 0.400 inch short in relation to the transmission input shaft, the clutch disc hub will ride on the input shaft very close to the end of the splines. While the input shaft is supported, this unusual location may create an additional load that could adversely affect the longevity of the pilot bushing and/or the transmission input shaft bearing. This condition is made even worse when using the Keisler bellhousing with its 0.200-inch increased depth. Buying a new Gen III/IV flywheel and then investing additional money in machine work may end up costing more than the McLeod flywheel. Given those realities, there are several reasons why the McLeod custom flywheel is the best overall choice.