Hi I've got an 99 SR model and the backbox is rusted to the point it is leaking. I'm wondering if I'm able to replace the backbox with a GT model backbox with it being able to just bolt straight up to the existing stock exhaust without modifications?

From what I can see they look almost identical but the GT exhaust looks a bit nicer

Here are two listings on Ebay:
GT: www.ebay.co.uk/itm/264395903883?ul_noapp=true
ST: www.ebay.co.uk/itm/254723597905

If so, will this hold true for aftermarket parts as well?

Thanks for any help.

im not sure. best would be to check toyota diagrams, but even then thats not a for sure thing as pipe diameter will be different

>

The pipe diameters are the same aren't they?

I think as a whole exhaust system it's easily interchangeable but as for individual parts bolting onto another system I'm not sure but I don't see why it wouldn't.

Chances are good that your flanges are rusted to oblivion as well, so nothing will "bolt on" because your flanges will turn to dust when you remove the existing muffler. I just had to go through this on my 96 GT - all the flanges were rusted and farting. I had to grind off old flanges and weld on new Walker brand universal flanges.
Toyota used what looks like stainless piping and even stainless welding wire on their exhausts from this era, but inexplicably installed mild steel flanges that rust away in time.

Diameter is a different size between the 7A-FE and 5S-FE/3S.