998 vs 727

If you are going 360 I'd do what AMC did and put a 727 behind it.
The 998 can be one tough transmission, light and strong, but a 360 is no slouch, and since you need to replace yours, why not do the upgrade now?

A little contrarian view versus other wise sages. Your 998 will be fine behind a mild 360. You can even beef up the 998 and not worry about buying a 727 which probably needs to be rebuilt anyway and it will save you $. 904 - 998 trannys get bad reputations mainly because of a poorly completed rebuild. My two cents of course. I have a built up 904 (more of a 998 now) behind a moderate 360 and I have had no problems at all. And I have stomped it hard. After all, many use them for racing with great success. But for full disclosure, I have a 69 Rambler and wanted to keep it more simple and lighter.

My first question is what is wrong with the original case? Other than the servo bores being damaged I cant think of anything else that would trash a torqueflite case. Maybe dowel pin holes if bolts were loose. A 727 is not a direct bolt in,the driveshaft  will need shortened and the yoke is different. Dipstick and tube are different. Unless my motor was making huge horsepower I would stick with the 998. I dont know which planet set this trans has in it, There is one that has a lower first gear ratio, lower than a 727 first gear. Gives the car a better launch, kind of like having lower diff gears. Its a lighter trans that uses less HP to run. Some will complain that the converter is weak, simple enough to use a 32RH pump body and gears. This trans uses flats to drive the pump not slots that cause the hub to crack. If your builder cant set this trans up for 450HP he needs to find a new job. Should not cost a arm and a leg.

I run the 998 behind my gremlin with a built 360 with no problems. running 5000 stall converter. Buy a good converter as that is the weak link.