Kyle is correct. The L67 is too strong for the stock transmission.
There is more to it than "top swapping". Yes, all the parts will swap over onto an L36, but your actually going to produce more power than a standard issue L67 because of the higher compression ratio. You would also need to tune the PCM after the top swap because the current PCM settings are too aggressive for the setup.
It would be easier to just swap out the whole engine/trans/pcm etc. Just like I did last year. Except I kept the original PCM and re-tuned it myself.
1997 Pontiac Bonneville SEi-SCRAPYARD
2000 Chevrolet Blazer LT 4X4 4.3L
2002 Chevrolet *-10 LS Ext Cab 4X4 5.7 V8