I've looked into this and i got a set of E7TE heads myself (my engine is dying so i never put them on, but i got them for free). The E7's have a smaller combustion chamber that brings the compression ratio up from 8.3:1 to 9:1 and I believe the intake port flows better. I've heard and read in books that it should give you a 10-15 horsepower/ft.lb. torque increase. They should fit with no problems, but as stated by Violent84 you should probably switch the valve springs, but they might actually work well for a larger lift cam.
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'84 Mustang 5.0 LX
My car