Wow, I didn't realise how little information there is on these engines judging by the silence.
At 3.5ltr they were fitted in a heap of ford Trader, Mazda T series trucks, Mazda T series buses both here in Oz and in Japan and I'm sure some made it over to your shores. My original intention was to do a 4BT swap but after finding what I can about these diesels it is lighter, non turboed produces more power, gains better fuel economy and is cheaper to buy in stock form. This one shown in my truck has done 500 000 reliable K's. A Perkin's workshop manual I recently acquired shows almost exact similarities to the Mazda 3.5ltr. so I guess they were built fairly tough.
It sounds like a great engine. I don't think it even made it to the US and since the majority of members are in the US we won't be much help. I'm sure Dougal knows something about them though.
1990 Ford F250 SCSB on RCLB frame, 4x4, ZF5, 7.3L IDI, 37s.
4BD1T looking for a permanent home.
I see some of those here too, but I have no information on them. The Ford equivalent truck was badged "trader".
There are factory turbo versions wearing the 3500T badge but that is all the info I have.
For your turbo sizing, the 4BD1T information thread will all apply quite well. Yes you are down on displacement a little, but the turbos I've picked there are all fast-spooling.