any advice on building a swing arm
My advice would be don't !
The thing to ask yourself is what exactly happens when one rear wheel hits a bump,
If the whole axle moves, that must mean the other wheel leaves the ground (bad)
If the whole axle does not move, that means the whole bike is deflected about it's longitudinal axis, which with four inches of trail at the front end will mean the steering is violently shaken (again bad).
Or if the one wheel station is to move without ether of the above then the swing arm has to flex, and that flexing will result in fatigue cracking.
I have had a number of 'swing arm conversions' come in the workshop,
and could not claim they were anything other than a very dodgy idea.
The Pan as stock has something like a 2.8 to 1 final drive,
so you would be better served by chopping down the rear end from a 328 or 323i BMW
mounting up the diff solid,
and using some 1 1/4 or even 1 1/2 thick walled tube to form two semi-trailing arms
to carry the rear wheel hubs.
plus I would recommend reducing the trail at the front end by at least two inches,
which will bring the steering effort down to something manageable.
Generally I could not recommend attaching an archaic beam axle to any kind of swingarm type thing, the small amount of extra work involved in making a simple IRS is more than out weighed by the better result obtained.