im Running a 400m in my truck and it haswnt been started Yet. i just want to get a few things straight. i just bought a 2bbl holly 500 cfm carburator is that going to be Big enough? can i rejet it to a 600 cfm im alittle confused on carb stuff. the motor is moslty stock at the moment besides. MSD INGNITOIN SYSTEM, NEW RINGS. MILDLY PORTED AND POLISHED INTAKE AND HEADS, SMALL CAM 509 LIFT AT CAM AND 218 DURATION AND 276 ADVERTISED DURATION. will i have to get aftermarket valve springs? becouse it says the stock springs only go to 480 lift at cam? or what kinda of valve springs should i use? please be specific.

 so theres three questions i need answered will the 500cfm carb be enough? can i make this carburator any bigger? and will the valve springs be enough to run the cam. Thanks guys PLEASE HELP lol.

new springs are a given. just cheap insurance. as far as the carb it really doesnt seem like you have done alot to it but the cfm rating cant be changed by jets.  buy a 600 cfm carb run it normaly for about 3 hrs (DO NOT HOT ROD IT) look at the plugs and then go from there on the carb choice. you throw a 700 cfm 4 barrel on a head setup for a 2 barel and i prommise you will waste more gas than you burn. personally i would go for a small spread bore aluminum intake with about a 600 or 625 cfm carb i have always had good luck with that setup

dont use holly go with 650cfm

you need new springs, but no matter what they say a 750 is WAY to big for a 400m mine ran best on a 450 4bl... but a nice 600 would fit that truck best

the 2bbl is fine cause most of the older trucks came out factory with them and they work good im stil runnin a 2bbl on my 351w in my bronco and u dont use near as much fuel as the 4bbl carbs but its all on what u choose is the best for what u are goin to use it for 

These people with the bigger carb is better, crack me up. I have owned these 73-79 fords for years with the 351m/400m.Along with the 460 BB and the 300 engines.With what youve had done so far the 500 wont give you what you need anymore.For your engine a 600 carb will be perfectly fine you could go up to a 650. I would NOT go higher than that. These Fe engines do not do well with anything over a 650 carb unless you really have major work to consume that fuel. Other wise you will be dumping too much fuel as in slighty more than the engine needs.The 351s and the 400s are exactly the same,EXCEPT the stroke on the crank,and the tops of the pistons. I cant remember if there was a difference in the piston size or not. The stroke alone changes the CI.My trucks always had a better throttle response from a Holley. But better gas milage from an Eldbrock. As for the valve springs,Change them. I cant tell you which ones though..Call Summit they can help you with that. Also you should consider running roller rockers with I would say maybe a 1.5 rocker,A 1.7 ratio might be too much with the lift on your cam.And you look into a set of Long tube headers,With what you have it would do you good.Hope this helps