http://www.vtwinmfg.com/Catalog1/VTwinCatalogIndex1.htm

Go to the above link, go to page 442 in the Classic Section. This will give you an idea of how MANY different flywheels were used by HD. I am not an expert on the older engine, my area of expertise is performance on Evo and Twin Cam styled engines.

Give a better description on what you have and what you want to do and I may be able to help you better.

EDIT: Cases:
From the sounds of things, you need to study up on HD engine designs a little before you attempt anything. You are asking what cases to use...FL, FX or XL. Those letter designations have nothing to do with cases, they are types of HD motorcycles. The XL is a SPORTSTER which integrate the engine and trans into one unit, which is no where even close to what you need. The FX and FL are desinations referring to Big Twin powered bikes. FL's are models with the big, fat front ends with enclosed headlights. The FX refers to models with "standard" type narrow-glide or wide-glide front ends. The FL's and FX's are all powered by Big Twin Engines that all use the same case.
Engine cases are engine specific, FL or FX are the same, XL is a Sportster and totally unrelated. S&S and STD are the 2 best case manufactures, so if you need cases, look into them. Use the link I sent and look through the catalog, you will see ALOT of useful info.
The 74" & 80" engines also use different bores and strokes. Naturally, the bigger the engine, the power it will make.
a 74" has a 3-31/32 stroke & 3-7/16 bore. The 80" has a 4-1/4 stroke and 3-1/2 bore.
Without knowing more detail about your engine...its condition, state of repair, whether its been stroked, has big bore cylinders, etc.....I don't know what else to tell you. Your best bet is to take it to a reputable shop and have them completely go through it and tell you what you will need.