I have a 75 ford f150 with a 76 grand marquis 450 big block. It's a drag truck. The problem is that when I try to mash the pedal to the floor to take off it backfires on me and won't take off.

uuhhhhh don't floor it? Sorry ..Seriously when you have an automotive engine with a carburetor in a so-called 'Drag Truck" - ,,, whatever that means - and backfires - fuel is entering exhaust stream unburned then igniting in exhaust. TIMING of distributor. Or please forgive me if you mean backfires back up in to intake... well never mind then your engine, for that matter your entire truck and you inside would be crispy critters.

Some folks call a stack fire a back fire and noise in the intake can be a lean condition that happens when opening the throttle. If it tries to blow the muffler off ? thats "backfire" A 460 is a Ford engine, there was no production 450. A lean condition can mean inadequate fuel volume, pressure, or delivery. A big block in a truck in production had a "hot fuel" issue that could be remedied on a little bit newer ones. since you built it, you may have engineered yourself an issue like that or simply have weak pressure or plugged filters. If you have the muffler trying to blow off issue, then start by checking tuneup and timing, timing advance as well as timing chain.

If your noise is in the intake, plan on some new gaskets. if you have a holley carb, yep definite gaskets and power valve. seek professional help. may even run nice with a carb swap to an AFB. Ask an old ford technician, he can tell you what holley pops and back fires sound like and listen to it.

Check the timing first. Also as mentioned above check the carb. It may need to be re-jetted. Dannyl is correct in that you likely have a 460. One other thing to check is make certain the plug wires are on the correct plug and on the correct location on the distributor cap. It is real easy to get #7 and #8 mixed up.

http://support.speedmaster79.com/customer/portal/articles/126520 5-ford-429-460-514-firing-order-distributor-rotation-torque-specs

Sorry I wasn't more specific but yes the backfire is coming from the tailpipe and it is a 460 big block*

Is your distributor positioned like the service manual drawing for it ? You can use chiltons, motors, or mitchell, or other manuals that have tune specs. Is the body turned as designed? Is the rotor pointing for TDC as designed at 0 Deg crankshaft position ? does the rotor point at #1 in the cap? please describe your ignition system, ? when it is at TDC compression stroke, observing the rotor in the distr. with the cap off, how many degrees can you turn the crank damper before the rotor turns? If you cannot answer then ask a technician to look and measure the dwell variation and timing chain slack. If you can still find a technician with employment who understands what that means. You may be better off studying the manual and answering yourself. Or, ask a dealer tech. sory, that's what you need besides fuel system tests.

The next step is to verify the wires on the cap and number them for the cylinders as specified in the service manual. are all the wires going in the firing order, in the direction specified , to the cylinders, numbered as specified? excuse me for being precise, but that is what is so basic about system integrity.

Next step, are you using motorcraft parts? what color is your cap ? Is your cap and rotor specified for the truck or for the car ? have you mixed the two ? Or is the distributor from some other application ? This is why I say, Ford garage for diagnosis. see what they say in the first 1/2 hour. Exhaust leaks? causes of unburned fuel ? plugs are what part number and gap? results of fuel system tests? what fuel pump? and what carburetor? This was all unanswered from earlier. Hope they find it quick, you need to tell them what you made it out of.

If it is a modified engine the Ford garage will have zero interest in looking at it.

au contreire , walking out to take a look at what you are doing with their product ? Listening to what you did to it? Hearing what the result was ? Pointing out what eludes you, but is obvious to them ? Was always held in high regard. When men were men, Fords were blue. you put your pants on one leg at a time? Everyone seems to say found on road dead. These guys say first on race day. If testing was required ? they could answer quickly, we always did. Or they can direct you to a local Ford guy with time and old service manuals. Our parts guys used to just love answering what fit and what did not. sometimes the reality must come from the technician to the parts people because the old book is still misleading. The correction may not make the new book. The new kids have black caps, hahaha

There is a lot of good advice here and like the others have said, if it is backfiring check your distributor firing order. sounds like a timing issue.