6-1/2 is stretching it a bit wide but I've done 6" successfully...the trick is wheel offset....
....MGB Rostyle offset is, roughly, 3-1/4" on the thinner rostyle wheel....if the other wheel has more offset, you'll have to get spacers to center it in the wheelwell...
I always fit the front wheels first...bolt the wheel tight & then test lock to lock to see if the wheel rubs on the anti-sway bar or steering link....if it does. go to the local aftermarket wheel place & buy 4x4-12" aluminum spacers to move it out enough that it doesn't rub (don't worry about the tire rubbing - I've got a tire that's 8-3/4" wide on my GRM car & it doesn't rub anywhere)...my 15"x6" wheels required spacers because of their backspace
...in the rear, first make sure the bolts holding the shock to the body are pointed so that the head of the bolt is in the wheelwell - (if the nut end is in the wheelwell, reverse them)....then, based on backspacing, add spacers if necessary to center wheel in wheelwell....if the wheel/tire combo is too fat once its centered in the wheelwell, you can remove spacers to move it closer to the body or roll the wheelwell lip with a baseball bat or length of pipe....watch the bump stop area in the rear wheel well as that's the part of the body that sticks out the most into the wheel well
...I wouldn't buy a wheel 6-1/2"wide until I'd put it on the car to see if it rubs anywhere...& you can always go with a tire that has a low profile to keep it from rubbing...does that help a little?