70D to 8T and back.
While I can't go into specifics about what will fit and what will not, you ARE looking at 2 graders whose development is around 20 years apart. The 70D is somewhere around 35-40 horsepower more than the 8T and maybe as much as 1 1/2 - 2 tons heavier. As Old Magnet said, your 8T is a dry clutch and the 70D is an oil clutch. The 70D came designed to have hydraulics pretty much standard as power blades (side-shift) and rear rippers were very common attachments on the later machines. Power steering design had gone through several generation changes and even the internals of the control gear boxes had been upgraded.
The transmission had gone from 2-speed reverse to 4-speed reverse and 4-wheel braking on the drive axles was pretty much standard as opposed to the standard 2-wheel braking set-up on the 8T. (You had braking on all 4 drive wheels on the 8T due to the connecting chains in the chain cases but brakes were standard on only the 2 rear wheels of the drive.)
If the engine is the only problem with the 70D, I'd be looking at the cost of fixing that. IF that proves prohibitive, then and only then would I be thinking of stripping stuff from it to put on your 8T. It may work better to look for a donor engine for the 70D and sell your 8T to your friend as a parts machine for his jigger. I doubt the 70D transmission would fit easily into his 8T for the same reasons that you are having trouble mating the 8T engine to your 70D frame and transmision.
Keep asking questions, M8. You'll get all sorts of answers from all sorts of people. Then, YOU have to decide what to do AND do it. All part of the fun. LOL.
You have a wonderful day. Best wishes.