Will the 88+ oil pans fit on an 84? I'd like to upgrade to the 01 piece gasket as mine is leaking. Thank you.

 

Yes, they fit. Just be aware whether your existing pan is center sump or rear, and be sure to get the same configuration. Otherwise you'll have to change your oil pick up and dip stick location.

Rather than ordering a new pan, you could remove your existing one and with a grinder remove the ridge around each bolt hole, and that will allow your pan to be used with the single piece gasket. That is a common mod.

 

As F-250 said, you have to grind off the raised ridges around all of the bolt holes. The 4 piece gasket used the ridges to keep the gasket aligned, and for torque/compression. The 1 piece uses copper compression rings built into the gasket.
A dremel and a little grinding wheel can make pretty quick work of it. Just be sure to get them as smooth as possible.

Otherwise, you can just order a new oil pan. I ordered one for an '81, and it didn't come with the raised ridges. If you get a new one, be certain that you get one with the correct configuration of the dipstick.

If your current dipstick goes into the oil pan, and you get a pan for an engine that has the dipstick in the block, you will have NO dipstick at all.
If you get one with a dipstick hole, and your current dipstick goes into the block, you'll have two. This, obviously, isn't as bad as the first.

 

Thank you Sirs