I have one in the cellar waiting to go onto a stubby R380, into an 88". The whole ensemble will be 4" longer than the original.
With a 109, you can move the gearbox back rather than the engine forward, or a combination of the two so you keep the front the same. Even with a Series 3 front, you need a slam panel for the bonnet catch! I am moving the engine forwards a bit and the gearbox back a bit - the exact amounts are yet to be decided! Obviously moving the gearbox means extending the handbrake pull and altering the high/low lever and 4WD plunger, but that is not difficult engineering.
Moving the gearbox means you would need a shortened rear propshaft, which is not desparately expensive. You can either use as longer front shaft or you can use aluminium billet packing pieces to extend it.
You will need a debender type gearbox so the gearlever ends up in a sensible place!
People have said that there are standard props from other models that might fit but having an 88", I don't know any details!
Sunny Jim