Series 2a onto Series 3 chassis

In theory yes, but may depend on how the Holden conversions were done. You can do some simple checks by measuring the distances from the engine mounts to the front cross member and the outriggers for the firewall. Check measure the distances between the front outrigger and the outrigger which has the two vertical mounting plates for the back of the cab. If all measurements are ok then its a simple bolt up job.

I dunno if this helps, but I recently put a SIII chassis on my 1969 IIA,

the only significant difference was the position of the boltholes behind the seatbox, 2 of which no longer lined up to the seatbox/tub. No big deal, I just left them out.

Everything like Bulkhead/wings/doors/sills should be interchangable, although note i know bugger all about Holden conversions and I was only fitting the bog standard pommie 2.25 petrol engine

Hope this helps