Thats a nice find you have there gumpit !! Its not a spider but a heavily modifed springer. Which whoever did the work to it definatly had experiance working them.
They moved the axle back to accomidate a duel jackshaft for the gearing. they use this for a couple of reasons 1) max gear on the driveshaft is 45 teeth untill your hitting the frame 2) you can only have so big of a diameter of a secondary clutch untill your either hitting axles, motor, spings etc 3) it puts your center of gravity up forward so your not having issues with it flipping over espcially with hard clutch engangment issues with a much bigger motor.
The mono leaf springs they use is a good idea because the original link desgin would hit on the motor casing.
here is a link to a unmodified frame
Untitled Document
I look forward to hearing how it turns out good luck !!