a little at a time
As donsrods says, a 1/16th" here, and a 1/16th" there. I would suggest you try to fit the panels that match up to the body structure first. Things that fit to areas that can't be moved, like that door to the A pillar gap, for example. Make sure the door is properly aligned in its opening. Look at the door to rocker panel gap. Make sure that gap is nice and level. And make sure the door to quarter gap is good. When these panels fit with their openings properly, it makes it a little easier to fit the more adjustable panels to them. If you start with one of these main panels off just a bit, you'll fight the rest of the panel gaps for days! Because you can't really move a door too much to fit the hood or fender cuz you've got a door opening to consider. Also, try fitting the hood to the cowl-(this only applies if the cowl panel is part of body structure, not a bolt on), If the hood to cowl gap is right, and the door quarter panel and door to A pilar gap is right, and you still have too much of a gap between the hood and door, then you are left with donsrods suggestion of welding more material to the edge of the hood to make the gap smaller-with a mig, not too hard to do, and a pretty common practice. Just build up a series of welds 'till you get it close, and grind it smooth...just my 2 cents worth-good luck!