Just wanting some questions answered if i can seems to be too wide a
variety of answers when i use google.
1. What are some engine swap choices i have with the 87 if i use the
original tranny ive been told i could fit a 350 in it if i got an adapter or a early 90s or 80s straight 6.
2. where is the transmission fill at
3. when i drive my raider it seems to have trouble when i hit hills and doesn't have any power on them.
how do i re set the speedometer

1) another 2.6 & the 2.6 turbo from a starion/conquest would work.
2) don't know. check your owners manual if you can't get an answer here quickly enough. It may be your dipstick tube.
3) Reset the speedo? What do you mean by that? To resolve power issues or other things? For the power issues, you should check the condition of your carb and confirm there are no vacuum leaks as the stock Mikuni is highly affected when they exist. Also, don't start immediately tearing out the stock vacuum lines w/out properly documenting what you're doing. Otherwise, you're bound (highly likely due to it's complexity) to screw up putting the lines back. If you do replace the lines, do them one at a time. Don Huysman did a great job documenting the carburetor setup with diagrams. I no longer have the links to those (I don't have a 2.6 so I'm not focused on remembering them) but maybe someone else who has them can post them up or post up a link to them.

1. What are some engine swap choices i have with the 87 if i use the original tranny ive been told i could fit a 350 in it if i got an adapter or a early 90s or 80s straight 6.

The engine choices are only limited by budget and common sense. The original transmission will not handle much more than the 2.6L, and no, there are no adapters to use a Chevy engine. The straight 6 would not physically sit in the engine bay, either.

There have been successful 302 Ford, 6.0L Caddy, 350 Chevy, and others that require lots of fabrication. The Starion / Conquest swap is easier and uses factory-available parts. The 2.4L engine in later Mighty Max / D-50 truck is a good choice for you, too.

2. where is the transmission fill at?

The transmission fill plug is on the left side of the transmission about 1/2 way up the side. Look for a large nut.

3. when i drive my raider it seems to have trouble when i hit hills and doesn't have any power on them. how do i re set the speedometer

You'll find that most 2.6L owners complain about power in the hills, especially if you're used to new V8 trucks or cars. When I worked in the mountains east of Sacramento, and into the Tahoe area, I simply expected to use 2nd gear on the climbs and figured myself lucky to use 3rd anywhere outside Placerville. I have a bunch of vacuum line photos from my '87 Raider when you have the need for review. Lastly, make sure the secondary diaphragm on the carb is working, otherwise you have only a small primary to work from.

That secondary not working will really make your truck drag. Look up the manual at mitsubishilinks.com, and also there is a manual for hopping up your 2.6, which is a good read.

Best unboosted choice is 4.3GM v6 with 4l60e or 700r4 with NVG 231 or 242 transfer case.
Best boosted choice is Mitsu 4g64 2.4 block with 4g63 dohc head and turbo bits and the stock a/t. M/T won't take the torque.