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What flywheel casting number and or size do I need to use behind a 235 that came out of a 1955 Belair that will be going into a 1958 Apache? I have the original 1958 truck bellhousing, but have read the flywheel and corresponding clutch was larger on the trucks than on the cars?

There is a flywheel that will accept a heavy duty clutch cover with nine attaching bolts instead of six, and a slightly larger disc, but finding that clutch assembly without paying to have a core item rebuilt is going to be a real challenge. Otherwise, just use any flywheel that matches the starter- - - -139 ring gear teeth for a 6 volt starter, or 168 teeth for a 12 volt, with whatever clutch that will bolt onto it.
Jerry

David, I'm not certain of the casting number but I have a flywheel made for an 11 inch pressure plate (and correspondingly as you would expect the 11 inch clutch). It is the 6 bolt style. These were used in heavy duty applications and the one I have was removed from a totaled 1954 Corvette. If you can find one, the larger size is nice if your pickup is being used as a truck. If your pickup is not being used to haul loads of any consequence, as Jerry said it won't matter. Interestingly the Astro Van my five speed transmission came out of used an 11 inch clutch if it was the heavy duty van with the V6, so the clutch plate that matched the transmission input shaft wasn't too hard to find. Good luck.

Some flywheel info.

Flywheel is the 168-tooth variant casting number 3705410. The other number cast into the flywheel is 31855 or B1855.

So, do I get to choose between a 9 1/2", 10" or 11" clutch kit?

Again, flywheel is off a 1955 Bel Air, using the 1958 truck bellhousing (for the proper motor mounting) and I assume use the truck throw--out bearing to match the clutch fork.

David, That casting number when searched through GMPartswiki comes back with multiple part numbers and applications for both car and truck. A 1970 Chevy Master Parts Book indicates that casting number under part number GM# 3789730 was for 1955-62 Pass cars w/11" clutch (6 cyl.). An earlier 1960 Truck Parts Book indicates it's listed for 1960 C10-30 - 6 cyl. (10" Clutch) as GM# 3705409 with the same casting number. Other books show that casting number for 9-1/2", 10" or 11" clutches in different applications. Just my $.02 worth, but take your flywheel and measure across the clutch disc mating surface as shown in an edited pic of yours below. That can be your guide to which clutch disc kit to use.

BTW - that "B1855" is the casting date code which would indicate February 18, 1955.

That flywheel can be used on both a car and truck with a 235 engine. A 9.5 clutch was used on a 6 cylinder car. Most 6 cylinder trucks
used a 10" unless it was a HD application. That appears to be drilled and tapped for a 10" clutch. An 11" would have the bolt holes
closer to the ring gear.
You want a flat diaphragm clutch with the long bearing. Most parts stores will try to sell you something else but
it won't work.
Fred

BTW. Not all Tri Five Chevy cars are Bel Air's. The car that was most produced and sold in those years was the 210 four door sedan.
There were also more 150 models sold than Bel Air's. A wide assortment of engines and transmissions was available for all models.

If you are buying a clutch kit, I recommend that you buy it from a local store. Buying online is great, but if you buy the wrong one, the return shipping is expensive. Might as well give it away.

Here is a flywheel for a 10 inch clutch and also one for an 11 inch clutch. I had resurfaced the 10 inch flywheel at the time the image was made. The 11 inch flywheel was as I got it...not resurfaced yet. You can see how much closer to the perimeter the holes are on the 11 inch flywheel. Hope it helps.

As to your question about the throw-out bearing, I would be inclined to wait until I purchased my pressure plate and clutch disc and see what length I needed...I have a flat diaphragm pressure plate and I had to use a short throw-out bearing and even then I had to make a new connecting link as the original one didn't allow enough adjustment room. That was also when the T5 was installed but I don't believe that had as much effect as the 11 inch pressure plate did. I tried the long to bearing but it would not work at all. Good luck.