Nitrous is a good idea, but if you want to stay NA, then your choices are limited. I did the small mod here and there routine and was routinely disappointed. I would say save just a tad more and get a converter. Mine made worlds of difference and was around $400 or so with the solid input shaft. There is not much else you can do if not doing heads/cam/intake. You could always get some 1.7 or 1.72 rockers. You could also get an intake, I know of a GP on the new Mac for like $300 delivered.
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1991 Mustang GT - Cobra Intake, FMS 65mm TB, C&L 73mm MAF, GT-40P heads, Crane 2031 cam, Crane 1.7rr, Built AOD, Art Carr 2400 non-lockup converter, Transgo shift kit, BBK chrome cold air, FMS 'P' headers, Pacesetter off-road H, BBK aluminum pulleys, 2-C Flows w/ dumps, 4.10's, Steeda STB, for now...
The Original GT-40P Tech Page