You do not have to remove the tensioner completely. All you have to do is loosen the bolt on the tensioner. It is a regular bolt with regular threads, they are not backwards, and you just turn it left and it will losen. You will not break the tensioner if you are turning the bolt the correct way. If you cannot get it to come off, the only thing I can suggest is to remove the whole tensioner.
But, what I really want to know is why you need an oversized tensioner. If I am correct you are running a 3.4" Pulley. Since you are running a 3.4" Pulley all you have to do is go to an autoparts store and buy a gatorback 4060715 gatorback belt and it will fit great, at first it will fit very tightly and you will have to put it over either the tensioner or flat idler first because it won't fit over the ribs, but it will stretch out after about 50-100 miles and fit perfect. Then you will get no belt slip, if you are getting any now, which would be odd. We don't get any even in the GTP.
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!