The crack in the 6.9l block was a cast flaw and was corrected midway through the 1985 model year. Even still, it's generally believed tha tyou can minimize the risk of a crack on an earlier block if you don't use the block heater.

Also, even given that, it's not all that common...and it is repairable without completely rebuilding the engine (as you would need to on a 7.3l that's succumbed to cavitation). For that reason, I'd actually take a 6.9l over a 7.3l IDI, unless I knew that the SCA's were kept up...

Just my $.02...also, I would venture to guess that most SRW box vans are 1/2 tons, and Ford didn't offer diesels in their half-ton trucks; the suspension can't reliably hold the weight. OTOH, switch to 3/4 ton springs and wiht a 2WD, you should be good to go.

IMHO you'd be best off either finding a Ford IDI or planning to swap one in. Then you wouldn't have to worry about custom fabrication or anything like that. Given the sort of strain the 617 would be under, I would venture to guess that mileage would be similar, as well.

Good luck!