where to start? - T5 upgrades

Nothing you can do without a better gear set and quality bearings. I had 150k miles on my T5 with 500 whp for 60k of those miles but I never shifted with the rpm up always let it drop a couple of thousand rpm to get it to last. It finally went and it was the 5th gear because of me squeezing the throttle and getting into boost. I have seen cars with new T-5s last a matter of months because of running Drag slicks and speed shifting with only 300hp so don't count on it lasting long with your 351ci torque. You can search every Forum and Site on the Internet and you will see it won't last. Heck GM did not even offer a 5speed on the 350ci because it would not last under testing so why have warranty claims when 230hp was just destroying the unit. It takes care to last and I am sure I was lucky that 5th was the gear and not 2nd or 3rd.

Sell the T5 on Ebay and save your money for a Tremec TKO 500 or 600. The TKO units are almost indestructible as long as you keep fluid in them and don't try shifting without the clutch. By the time you rebuild the T5 a couple of times, you will have paid for the TKO.

The t-5 probably won't last too long behind that much power. My last Mustang had a 351W in it with a TKO behind it and that tranny took a beating.

Have you looked into a G-Force gear set? They are a bit expensive, but will hold up to most anything you throw at it. 450hp + slicks = blown third gear on a stock T5.

Check these guys out if you haven't already

http://www.astroperformance.com/product_info.php/manufacturers_id/29/products_id/207

http://www.g-forcetransmissions.com/tran_gt-5.asp