If you are going to toss it when you are done and if normal wrenches do not work a pipe wrench or chain wrench might do the trick.
If there is not enough room get a wrench on it you can remove the whole supply pump; there are 3 nuts with washer holding it on. You can put the supply pump in a vice (do not grip it any where near the mounting flange where the three stud holes are drilled as you will break it). Grip it on the main body. Another way would be to squash the primer pump assembly in the vice and use a wrench to turn the supply pump; again staying away from the thin flange where it mounts to the IP.

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84 300D, 82 Volvo 244Gl Diesel