Ive read a 496 is wider than a 454. Is the block wider, or are the exhaust manifolds wider?

496 is a tall deck (10.250") block so with heads it measures about 1/2" wider per motor at the exhaust ports. As for exhaust the Vovlo exhaust manifolds are the old Volvo BBC style so the overall package would be about 1/2" wider per motor or 1/4" per side x 2 if your mounting twins. The 496 mercury manifolds are an inch or so narrower. I don't think any of the builders made any real changes in the engine compartments or drive locations when they switched to the new 496 engines.

Ray @ Raylar

I replaced a 454 with a 496 in my boat. The mounting locations were exactly the same but the exhaust was wider by about 1/2 inch on each side. I thought at first I was going to need to cut new holes in the transom but I was finally able to connect to the tips using some tailpipes with flexible couplings. Going to be problem if I ever get a set of headers though.

I plan on ordering 496 volvos for a repower. Would i be better off using my current stainless marine exhaust? I would like to be able to retain my gil silent choice diverters.

Absolutly!
Ray @ Raylar

I had height issues when I went from a 454 to 496. 496 is a few inches taller.