Well your on the right track, check out my combo, mine consist of all stock heads, cam and intake, but to achieve 300rwhp, you need more than what i have, Heads you want will probaly need to be iron heads if your looking for a low end powerband, windsor sr.'s are a good choice, and have a 202 valve, one more thing that i recomend every stang have, is an electric fan, some folks here disagree and say they overheat, i have never ran cooler to honest, and i felt the power gain, probaly cuz my old clutch fan was worn.....good luck, you should receive much help from everyone here.
oh yea, rear end gears are the best mod for begginers, you will stay in the powerband where your car makes power, but depends on how much higway driving you do, and which gearset you choose.
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Police package 5spd 90 Lx, Stock original motor, 3.27grs, BM fan, fms 10.5 clutch, D&D quadrant, and adjuster, ADS chip, 180 stat, mac cai, mac h-pipe, mac subframe connectors, ASP crank pulley, ripper shifter, 26x10.5 M/T sport pros welds. 246 hp according to analyzer 
Race weight: 3,120
E.t-13.57 with 26x10.50's e.t. streets. 1.88 60'
mph- 99.92 mph
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