ultimate street 318
I would definately just drop a stock 318 in and weld up a turbo kit myself. I did this on my '77 W200 Powerwagon 3 years ago and it runs well. I bought a second hand (though it was still brand new and in the packaging) Holset HX50, took off the headers and bolted stock truck manifolds to the motor and welded up some old exhaust pipe I had laying around. At first I had locked timing and just recently have switched over to a boost retard CDI, but the whole project has cost me under $1000 and if I grenade the stock 318, there's more in the shed to drop in. Cruising the highway you get the manners and mileage of a stock 318 and with a push of the go-pedal, you're out in front of the rest.
I personally won't ever build a naturally aspirated motor again. For more power, I'd build a solid motor, but will always go with turbo's.