For all the trouble you might as well go with a 606 turbo.

The blocks are physically similar. You might have to go with the SDL oil pan for crossmember clearance and SDL oil filter block to keep the SDL oil cooler. That might mean a custom turbo drain pipe. Shouldn't be too difficult to fabricate a custom downpipe if the turbo outlet is in a different location. Getting the right radiator hoses might be a challenge but how difficult can that be?

Definitely run a 603 IP so everything is where it should be. Possibly a loss in power output since a 603 IP isn't calibrated for 175hp or whatever a 606 turbo puts out. Hopefully the 606 glow relay doesn't need a 606 EDS to function.

Do 98-99 E300s have a 722.4 or 722.6?

Sixto
93 300SD