JMC no longer makes swingarms, it is now obsolete and there is little information on installation.
Abbey Engineering reportedly bought out JMC's swingarm line and employed staff from JMC to manufacture swing arms under the JMC name in the North West of England. But they aren't making swingarms any more either now. ¹⁾

But you can still find these swingarms for sale second hand.
These were sold with either slotted chain adjusters or eccentric adjusters. If you wanted you could have the swingarm with additional bracing.
NOTE from the CAS4 website:
The Eccentric Adjuster is standard on all the JMC swingarms (they sold). Both the Pro-Street (P-35) and (P-115) were special order options.
There were fitment notes in regards to the WIDE TIRE applications that are not available now. Offset Kits may be required for wider than stock tires.

The benefits of this swingarm is it's lighter and stiffer than an OEM swingarm (meaning better handling). Also they look good and a very well made. ²⁾

1989 XL883 with JMC Swingarm. ^3)
4)
* Part numbers for Sportster swingarms per CAS4 website: ⁵⁾

• (JMC-A008002) - Fits 2000 - up Sportster Style Frames, Eccentric Axle Adjuster.
• (JMC-A008000) - Fits 1999 and earlier EVO Sportster Style Frames, Eccentric Axle Adjuster.
• (JMC-A007000) - Fits Iron Head Sportster Style Frames, Eccentric Axle Adjuster.
• Additional parts that were available as a special order option: ⁶⁾
• (JMC-P35) - Axle Adjuster.
• (JMC-P115) - Axle Adjuster.
• (JMC-200-WIDE) - Up to 200 series tire application (180 wide with belt).
• (JMC-EXT) - Up to 6“ extension available.
• (JMC-ECC-TOOL) - Eccentric Axle Adjusting Tool (2 work best).
• Features: ⁷⁾
• Lower Your Bike for Cruising.
• Raise your Bike for Handling.
• Direct Bolt on With no Modifications.
• Add 2 Horsepower in Power to Weight Ratio.
• Made of 7020 Light Alloy for Superior Strength.
• Show Polished and Ready to install.
• Race and Bearing Included.
• Accepts up to a 160 Tire Under a Stock Fender.

• Help for installation:
  • Chain Adjustment. The benefit of the eccentric chain adjusters is that chain adjustment is simple. Just undo the pinch bolts on both sides and rotate one of the eccentrics using the holes supplied to turn it. The other side will automatically change until the tension is correct. Then tighten the pinch bolts and job done.

Pics below with a 1989 XL883 by Dave83xlx. Click on a pic to enlarge:

Pinch area on this 1989 XL883, the left side was 0.239” and the right side was .204“

• Eccentric Adjuster Pinch Bolt Torque. Since there isn't any instructions floating around, the torque on the these bolts has been a concern. The problem is you need the pinch bolt tight enough to stop eccentrics from moving under torque. But you don't want so much you risk causing pressure to crack the alloy arm. So Dave83xlx of the XLForum did some testing ON A USED SWINGARM to find the results below using the existing bolts it came with. ⁸⁾ The pinch bolt original position was marked on both sides of the arm. The bolts were removed and re-installed using a “beam” type torque wrench to tighten the screws back to their original position at the marked line. The result was 30 ft-lbs (40 nm). The bolts are 10mm with course threads. With the actual manufacturers torque spec not known, this is only a recording of the torque setting used for THIS swingarm. Use your own best judgement when deciding what torque to use.
  

App. Dims on swingarm for 1989 XL883: Click on pic to enlarge.

• Reviews
  • Some people argue that turning the eccentric adjusters moves the axle, changing ride height. But this is minimal and also it could be argued that slotted adjusters change length of bike. In my opinion, it is a good swingarm but there are some negatives. Technical information about these arms can be hard to find. Also it could be hard to replace things like eccentric adjusters if you damage them, being ham fisted. Please note I'm not a expert and these are only my opinions. ⁹⁾