So here I am, with a smoking Volvo and I need to replace the turbocharger on our 1991 740SE with 190K miles. First question for the board,
How easy it is to replace?
Are there any tricks/secrets to complete this task?
Anything special to consider or do when tackling this project?

We are planing on keeping the wagon for a couple of years, is it suggested to buy a remanufactured turbocharger? If so, any suggestions as to where (preferably in Souther California)?

The markings on the turbo charger currently installed in our Volvo are:
TD04H-13C S/N 043026
Part Number 3517646
Mfd. No. 49189-01000

I am told this Mitsubishi turbo charger can be replaced by a Garrett T25 series any specific modifications for this?

Have you considered oil being trapped in turbo as per following:

Excerpt:

"PROBLEM AREAS

A common problem with a turbocharger is oil passing by the oil ring seals, leaving a cloud of blue smoke coming from the exhaust. The oil drain inside the unit could be plugged with coke/burnt oil deposits, making the only route for the oil is to dump into the exhaust. Oil must be drained away from the turbo faster than the oil being fed to it. Consider the possibility that excessive oil pressure/volume might be being fed to the turbo, causing the bearing housing to be flooded with oil and the turbo to leak."

http://www.brickboard.com/FAQ/700-900/EngineTurboRebuilding.htm
--
Norm Cook Vancouver BC; 1989 745T 202,000KM

TurboCity on Katella in Orange did a good job rebuilding my 740 Mitsubishi Turbo. It's gone about 30K now without any problems. If you're handy they have rebuild kits and instructional videos available to purchase.

I would suggest replacing the turbo cartridge with the same model Mitsubishi cartridge. The cartridge can be removed without removing the exhaust manifold. It is fastened to the turbo afterbody by a clamp and is keyed radially so alignment is no great problem. Your old cartridge has a core value to a rebuilder (you should be able to find a rebuilder in your area via Google). In order to guarantee the rebuild a new oil feed line will be specified to insure it is clean of any carbon. The most difficult part of the job will be crawling under the car for the oil drain line and the lower support bracket. All in all this is not a difficult job especially with the Mitsubishi. Besure to replace the oil drain line gasket and the copper washers on the oil feed and water lines.