So, as I understand it, you want to weld your centres into the new 6" wide rims 1/4" to 1/2" further inboard to get a wider track. (I always get "negative" and "positive" offset confused.) I normally think in terms of backspacing and what you want to do will actually increase the backspacing a bit. To maintain your original backspacing you'd need to change the offset by half the difference in rim width, which would be 3/4", in order to maintain the same clearance inboard for suspension and tierod ends and bodywork. By the same token, spacing the rims out too far could cause rubbing on the inner fenders and fender lips. (I have a 3rd Gen IROC that has wheels with about 5/8" difference in offset back and front. A tire shop put the wheels on backwards and the tierod ends knocked off the inside wheel weights right away, but it could have been worse.)

While setting the rims out further isn't so important in the rear (other than clearance), it could cause heavier steering in the front if you don't have power steering. What you could do is space out the existing wheels with washers to check clearance (but not drive like that). Seems to me it would be a lot less work just to find a set of 740 rims in a junkyard, which are 6".
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1992 745, >500k km