stock side pipes rob over 100hp

[PSERRA]

I got classic roadster and i recently built an 426 stroker windsor engine tuning the engine on the engine dyno we made 605hp , i'installed it in my car and and considering the car only weigh's 2500lbs the car should feel like a rocket but it didn't so i brought it to chassic dyno and did three runs it was only making 359hp to the wheels i did'nt know where all the power went to. then i dicided to take the side pipes off and see what it makes, what a shock from 359hp to 509hp to rear wheels just with removing the side pipes so i bought set of proflow race mufflers and installed them and went and did another run and it made 484hp and 468tq to the wheels know the car feels like a rocket but i put a set of slicks on the back and i have 8.8 rearend and t5 transmission first launch i broke trans and rear end. can any body tell me waht i should be putting in as trans and rear end and what should this car run in the quarter mile ! p.s I'm just curious because i couldnt even make one run.