These kits will not cure a bad-fitting hood. The fender to hood and hood to cowl gaps should be good when you start. These can be adjusted to some extent. If you are putting this kit on a truck that is together and you're not happy with the gaps, start with checking the rear cab mounts. Worn-out cab mounts will let the back of the cab sag and open the gap of the hood and top of the cowl up. To adjust the fenders and cab to the hood, loosen the bolts for the fenders at the front air dams, lower valance, at the top of the fender/inner fender mating point, and at the firewall. Also loosen the fender at where it bolts to the cowl, behind the kick panels. This will allow you to move fenders forward or back, with the air dam bolts loose, you can roll the fender up the air dam a bit more to close up the hood gap.
Large gaps at the top of the back of the hood and the cowl of the cab can sometimes be caused by the front end install. If the truck had a Camaro, Volare, or some other subframe-style front clip installed, the front radiator mount crossmember could be off slightly. Shimming the radiator's U-shaped mount can help close that gap sometimes.
If you are moving into paint mode after doing the flip hood kit install and fitting the frontend, then take a few minutes and drill a few strategically located 1⁄8-inch alignment holes. This way all the fitting you have just done won't be wasted. If you drill a few realignment holes now, when you reassemble the sheetmetal after paint you can get the pieces back in the same spots.
I drill a couple of holes at the back of the fenders where they bolt to the cab. Then drill a couple, top and bottom, through the front air dams where they bolt to the radiator mount. Then drill one on each side under where it will be covered with the plates that mount the fender rods. Those holes ought to get you right back where you were when you reassemble. We have done them this way for years and it really speeds up reassembly of the front end and hood. Follow along with the pictures and see just how easy a flip front hood can be to install.