I assume your '70 'Stang has a '70 vintage 351C 4V (M code), if so you've got the good heads, the D0AE closed chamber heads. Set 'em up with one piece valves (as somebody already mentioned) new retainers and keepers, and the springs recommended for your cam grind. Since budget is an issue, we'll skip the port/seat/chamber work and leave 'em as is, however some work to improve low lift flow would be nice if you make the budget for it. Consider a set of Ford's bolt down, pedestal mount roller rocker arms (M-6564-C351) a must. These require no headwork to install.
Hopefully a rebuild of the lower end is within budget, with good rod bolts, Clevite bearings and forged flat top pistons. Pay attention to the cylinder finish, and hand fit the rings. The amount of bhp you'll make is directly proportional to the amount of ring seal you have. And please, don't run a 38 year old damper, buy a good quality new one (Romac). A 38 year old damper is a ticking time bomb, waiting to explode. I wouldn't bother having a M code damper rebuilt, as its rather wimpy. Dynamically balance the reciprocating assembly.
Since a roller cam is more expensive than a flat tappet cam, I recommend a flat tappet cam for budget's sake. You haven't mentioned how you're using the car, "agressive cam" is a bit vague. I'll assume you want to retain the low maintenance hydraulic lifters, and that drivability and relaibility are important to you. The Comp Cams XE274, a hydraulic flat tappet cam with a rated power band of 2000 - 6000, 230/236 duration at 0.050", 0.562"/0.565" lift fits the description pretty good, and makes enough vacuum for power brakes, etc. Make sure you use oil with sufficient ZDDP in it, like the Valvoline VR1 racing oil, in order to keep that flat tappet cam from failing. Spin that cam with a Roll Master timing set.
Top this long block off with a '72 vintage spread bore intake (no egr), which you can pick up cheap. Bolt on a rebuilt Autolite/Motocraft 4300D spread bore carb (about 750 cfm).
For a fuel pump consider the high flow pumps made by RobbMc Performance:
http://www.robbmcperformance.com/index.html
Install a Duraspark distributor from a 1977 vintage 351M, 400 or 460, with the Duraspark 1 module (red strain relief) and an electronic ignition compatible coil. I can provide more info about the module if needed. You'll run around 12 to 18 degrees initial advance and re-curve the dizzy for 20 to 26 degrees centrifugal (depending upon how much initial advance you dial in) for 38 degrees total in by 3000 rpm.
The headers have already been discussed, you'll need a free flowing exhaust behind those headers.
That leaves us with the oil system. A good oil pan is the best insurance, something with high capacity, baffles, scapers and a windage tray all built-in would be nice. A standard volume oil pump with a high pressure relif spring, and an oil filter designed for higher pressures (Fram HP1, Motocraft FL1-HP, NAPA 1622, Wix 51515R).
Rock and Roll
George
