With your current mods, I doubt that the stock throttle body is limiting your power. Your maf may be, so a larger unit there may be helpful, but I wouldn't count on any performance increase from a t/b upgrade with your current mods. If you are preparing for the future, I'd go with a 65mm. The 65 mm unit will support 375-400 HP at the crank without being a bottleneck to flow. Even with heads/cam/intake, the majority of normally aspirated street 5.0L's out there don't flow enough to outrun a 65mm throttle body. There's also no bottom end loss with an oversized throttle body. Drivability may suffer a bit - since it's harder to modulate small amounts of air (part throttle driving conditions) -- the bigger blade means it takes less of a throttle position change to change engine speed.
If you're preparing for the future, the 65 will likely be all you'll ever need; and neither 65 nor 70 will probably help you right now as the stocker's capable of flowing what the engine needs right now. If you want to focus on the next bottleneck, it's likely your intake manifold. The stock throttle body will outflow the intake - so bolting a larger t/b on the stock intake doesn't make a lot of sense. The trickflow you mentioned is on target, or, for budget minded - look up tmoss. He does a great, inexpensive job porting the lower intake (stock upper flows pretty well) for noticeable power/torque gains - for great average power increases across the power band.
i agree with mike....right now i would spend the money on a MAF...also if u are gonna get the TB i would go with a 70 IF u plan on doing heads etc etc etc...even then alot of people stick with 65mm .. so its really a coin flipper.
Thanks for the info guys. the reason I asked about TB was I'm going to do mass air TB and intake all at once. I've decided on trickflow street intake, 76mm mass air,and from what you guys told me a 65mm accufab TB.
I just added an edelbrock performer and a 70mm TB. I do plan on adding a power adder someday, and only wanted to buy one once. I don't suffer any driveability problems. Still idles like a kitten (probably because I still have the stock heads and cam).
I picked it not only just for not having to upgrade again, but because it's matched to the inlet size of the intake.
Consider the Ford Racing throttle body - price is less than Accufab, not as pretty, but operationally they perform flawlessly, and they come with a new IAB solenoid and a new tps sensor. I think it's a better value unless you just have to have the Accufab. (poet)