will a 354 gear fit in a carrier that has 410 gears the rear end is a 1970 dana 60

Yes sir it will
3.54,3.73, 3.91 and 4.10 gears will all fit on the early 3 series carriers, posi and opened

What about 4.88s? I have an Ultimate Dana 60 that currently has 4.10 gears & I have a contact who will swap his 4.88s for my 4.10s. Are they going to fit?

To the best of my knowledge no...

There are spacers available but not recommended for manual shift or high horsepower applications.

However the 4.10 is not going to work in your friends car either unless you swapped carriers as well as gears.

Most 3 series carriers or spools are from 3.54 to 4.10 and the 4 series are from 4.56 to 5.38. Some after market 4.30 ratio gear will fit either one as long as you order the proper part number for your carrier
Dana Corp. use to only make one offset on the all of their Dana 60 ring gears so you had to change carriers to switch to the lower gears, Dana started making Dana 60 gears for both series some time in the mid 1990s is what I have heard recently
How to tell which part number for which carrier is the hard part
All the Dana 70 I've worked on are that way so you can order any ratio for any year Dana 70 carrier

I have used a spacer in the past with no problem , just use on lock tight on the bolts and proper torque.