I've searched the crap out of this and its been covered a lot, but can't seem to find anyone who has actually done this.  I put a 3.4L in 1988 4runner with an R150F from a 91 pickup.  The truck has 31" tires with 4.10s.  It turns close to 3k rpm on highway at 75mph, and I only get about 16-17 highway mpg. 

from what I have found, the r452 overdrive gears are toyota PN 33336-25020 for main shaft and 33046-25020 for counter shaft. It looks like there is a US dealer that has them:
http://toyotaparts.bochtoyotasouth.com/oe-toyota/3304625020
http://toyotaparts.bochtoyotasouth.com/oe-toyota/3333625020

The price doesn't seem unreasonable.  If I buy these gears, will they fit right onto the R150 main shaft and countershaft?  I looked into just regearing the diffs, but 3.73 gears for the front IFS are hard to come by.  I'd have to pay about $600 from the dealer, or find used gears out of a celica or other car that has a 7.5" rear diff.  I'd rather keep the 4.10 axle ratio, lower first gear in the R150 and just change the overdrive for highway mpg.  The current gearing is great around town.  Its got plenty of nut and I get 15-16 mpg city driving.  I'd hope that lower cruising rpms would give me a few more highway mpg. Any thoughts?

3000 rpm at 75mph is exactly where it should be. If you put a taller gear in you'll be falling on your face when you hit a hill or a good headwind hits.

I've just done this.  All the info is in a forum post I did on an Australian 4x4 forum:
http://www.4wdaction.com.au/forum/viewtopic.php?f=72&t=156994

Thanks for the link!  That's exactly what I was looking for.  Once the snow melts and I get my FJ40 back on the road, maybe I'll yank the tranny out of the 4runner and swap out the overdrive gears. 

So I am doing this. I already ordered the gears, and have pulled my stuff apart on my tranny. I am currently in the process of rebuilding my dual cases, and had everything down to the tail housing on the tranny anyways I have to wait on an oil seal for the 3.0 adapter extension housing (UGHHHHH) so ya.

I did run in to an issue though. I am unable to get my puller behind the gear on the output shaft... AND I didn't take the springs out or pulled my shift key's off the gear thing lol. it all slid off the counter gear assembly easily. after pulling the splined end and the synchro. I could remove the shift fork and whatever that shift ring thing is... all as one unit. I put it all on and off numerous times to make sure that it would go back on when I got the new gear set.

So right now everything is off except that top gear on the output. I guess I could take the bearing plate off, but didn't want to. Also.. I have a 90 r150f. And my 5th gear clutch fork is a separate piece like what AE86levin had in his pics. So I didn't have to remove a roll pin. the fork has a set bolt on the shaft that the reverse fork rides on. very simple.

Just saw the posts about the 5th gear removal problems.  Someone else over here in Aus asked me the same thing when they recently tried this mod. It seems the older R series that has a nut on the end of the shaft rather than a circlip that also has a bolt instead of a a roll pin has the 5th pressed on.  Mine slide on / off by hand so I had no such problems.  I'll ask the guy here how he managed but he had his out of the car.

I've just put mine back in the truck.  So I was logging in to post the results.  It runs great and I'm seeing 2200rpm at 100kph (61mph) which is our highway speed limit over here.  Also note that my Hilux is a '3L' 2.8 Diesel which I have Turbo'd without the Turbo it would be extremely under powered at 2200

:'( well the counter gear is the wrong one... I received the gears today. Dang it!!! apparently the newer r series are more different than we thought.

Not sure if I will be able to do this with my old pre-95 tranny.. DAMNIT!!!!!!
I should be able to get a used tacoma tranny and put it in my 4runner by replacing the bell housing I think. Then the gears should fit ya?

Looking at the various parts diagrams. I think there is a difference between the US r150f's and the rest of them in the world? lol. because ALL of the ones I see from 95 up to 2004 are like the one I have I think this mod is not going to work for us.

I always thought Marlin had done this in years past?

I'm running into this problem as well. I have a 92 r150f and #33046-25030. When the two gears are laying flat on the table, the Dyna gear is considerably taller and the bearing pocket is also bigger. Has anyone found another dyna part number that works for the older r150f's yet?

Hi guys
Long time sine I've checked on this but have had some responses on other pages and will update here also.
The original 4wdaction thread is updated so check that too, but the relevant info that *may* help people with older boxes is this.

I found this interesting info on a Kiwi forum that may be the answer:
http://www.offroadexpress.kiwi/Forums/viewtopic.php?f=13&t=27458
"ive run into the same problem.
i have a r150 from a 94 surf which has the nut on the end of the shaft not the circlip. standard fifth gear teeth counts are 26t & 46t dyna gears are 23 & 49.
i ordered dyna 5th gears from amayama, part numbers 33336-25030 (on mainshaft) and 33046-25030 (on countershaft)
the main gear fits but the counter gear doesnt. ive found more part numbers for the counter gear and think 33046-25011 is the one i need but amayama cant supply it.
it seems 25010, 25011,25012 are all the same counter gear for pre 1995 & 25020 & 25030 are for 1995+.
im trying to find a supplier has anybody accomplished this mod yet?"