just startin w/the mods!!

hello all! new here! i picked up a 78 f250 400ci auto w/66k miles on it! i am the 3rd owner! 2nd owner was a buddy of mine that put some new 33/13/16" tsl's on it w/ a 2.5 swamper lift&swaybars, new dual exhaust system, new brake lines and brakes, blaupunkt cd plr w/sony 6x9's! pd $2000 for it! anyway it has the towing package on it but seems to stumble quite a bit and stalls after running any distance/time of over 15miles/20-30mins! i am wanting to replace the intake manifold and 2 barrel carb. any suggestions from you guys and/or gals? planning on further mods in the future! headers, cam, lifters, pistons etc! any suggestions or comments would be welcomed!! thx scott

I have an 81 w/ 400M...I live in East Tx...am parting this truck out except for the trans...it has a new 2bbl carb on it but everything else is stock...