Sounds like maybe you aren't in US? Starting in 1969 US destined beetles all had IRS transaxles. Are you not using a VW pan - if so then go with what it comes with. You can convert it to IRS. VS made mostly swing axel Speedsters and convered from IRS to swing axel. Swing axle cars have a narrower rear width (track) so different ring and tires fit under the rear fenders. Here is a table that shows serial number and transaxle features (as produced).
A front strut Super Beetle chassis can be used. When VW pan is cut in half, use the front piece from a std bug. Or carefully remove the SB front bits and weld on the std beetle front pieces (for the std beam) that are readily available (must be aligned properly before welding.
Or find the old (rare) CMC SB conversion pieces.
Or go with a custom ACME frame or Coolryde's Mendeola front end -