well, everyone's pretty much laid it out for you. there arent any other than the ones you've already listed. and unless you plan on making 600+ hp, there's no need for an upgrade. with the money you would spend upgrading intakes, you could spend that same amount of money on other mods such as gears, cams or longtubes.
im not trying to be a dick, but if you ask anybody with a 4.6 n/a engine if they would rather have the bullitt/p-51 intake, or gears, cams, and/or LT's, im willing to bed the majority would say the latter.
now, i know you arent considering one of these manifolds because of the cost. i know you are looking for more inexpensive alternatives. but, just in case anyone reading this may be thinking about dropping that much on a manifold, lets compare. now, im not trying to badmouth the p-51 or bullitt, god knows i'd get one if they were more inexpensive, and i probably will when i decide to s/c....wait, i want a KB
. but they are good products made by good companies with good reputations. but if you're staying N/A, i would take a different route, and save a new manifold for last if anything.
this isnt exact, but for ****s n giggles:
P-51 intake: $1300
FRPP 4.10s: $200
VT cams : $600
BBK LT's : $500
now of course you have to factor in installation and tuning costs for both, and also upraded springs for the cams. but im willing to bet with the latter 3 parts you would notice more of a difference. anyway, there arent any aftermarket intake manifolds for the same price as the old 5.0's, and there probably never will be.