As was already said, you don't really need to upgrade. The 4t60e can be built up to be almost as strong as a built up 4t65e-hd. The stock tranny in my Turbo GP is the 4t60, and from the factory it came built up to handle the extra power of the turbocharged engine, and I can't think of many people having problems with them even in high mileage, fairly modded cars. These cars put out probably 200+hp to the wheels stock, through a 14 year old tranny with 150,000+ miles on it, and have no problem doing it. So, this leads me to believe that you could build up your tranny to be able to handle the power you are going to be making pretty well. I don't know how much power you are planning on making, but it would have to be A LOT to not be able to build your tranny for it.
1990 Maroon Grand Prix STE McLaren 3.1 Intercooled Turbo, 14.695 @ 94.49 on 12psi before cam/heads/etc.
1990 Bright Red Grand Prix McLaren 3.1 Intercooled Turbo, 14.819 @ 92.72, 189whp
1999 Grand Prix GT 9.75:1|K&N|160 tstat|No Ubend/Cat/Res|UD Pulleys|Ported M90/TB/Heads|XPZ|2.55"|Pacesetters|Stage 2 I/C. 12.442 @ 112.78
2000 Blue Bonneville SSEi Supercharged Gutted Airbox|K&N|3.4"|1.9'*|WP Pulley|Custom DHP|180*stat|41932'*|No Cat. 14.681 @ 94.80
2002 Dark Cherry Grand Prix GTP Supercharged & Intercooled
12.084 @ 114.38 w/1.743 60', 315.6whp, Fastest Stock Cam M90 GTP!
NOW XP CAMMED, 340.6whp with XP, stock heads, Gen 3 M90, 11.984 @ 115.84 w/1.750 60'!
Most HP Stock Heads Gen 3 M90 GTP!
2004 Red Monte SS Supercharged K&N|3.4 Pulley|1.9'*|180*stat|41919 plugs|3" dp|No cat/ubend|DHP|Shift Kit. 14.2 @ 98
GOT BOOST??? Friends dont let friends drive stock!