Settin mufflers under the cab good or bad?

[old_1966_GMC]

So im havein some problems with my dad in where i should put my mufflers so first off i had thougth of puting my mufflers under the cab sence the frame is straigth and make 2 holes in the rear crossmember where the 2 drive shafts connect and have the exhaust tips comin out in front of the rear wheels but this where im have trouble my dad says its not good to put the mufflers under the cab on the frame because if theres a leak it not good to breath the fumes.so what u guys think will there be any problems installing my mufflers under the cab or should i take my dad advise in finding a diffirent place to put my muffflers i mean i really like the way i had n mind but i dont know what to do now PLZ any help or photo would be helpful
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