Having literally just fitted a stainless steel exhaust to my 109 1982, try the following:
Front wheels on ramps or stands.
Pass the rear end of the front pipe up and on top of the gearbox
Turn the manifold end towards the front off side wheel
Wiggle the back of the front pipe over the geardbox mount
Tip the front end so it touches the floor and push the pipe back whilst inserting the manifold end up twixt the engine and chassis
I have spent all day faffing about changing this ****. It looks like it was designed by a bunch of 17 year old tech college students with the design brief "make an exhaust system incorporating as many twists as you can" and the caveat of "if any of you consider ease of fitment you will automatically fail yer O level metalwork."
Jesus H F***ing Christ!!! But I got it on. Got no knuckles, no f***ing sense of humour and ache all over. Good luck.