TF 999/998 sporting internal diff from the 904 ?

A-998s Were not ALL loc-ups. On the top of the bellhouse behind the Distributor, you should find cast into the bellhouse , the call-outs A998 or A999
I have one each A-998 non-loc-up, and A999 loc-up. They are wide ratio transmissions and work well with super-high hiway gears, behind stock hi-torque low-rpm engines, and adapt well to slightly juiced up SBMs . But for big-cams, you still need to keep pace with compression and upgraded rear gears and or a higher Stall TC, same-old/same-old. For streeters I recommend to concentrate on what second gear is doing.
The ratios are 2.74-1.54-1.00,and splits are .56-.65;(versus 2.45-1.45-1.00 and splits of .59-.69 in the regular A904).
With 3.55s this makes your roadgears 9.73-5.47-3.55....... so the stock 112LSA cams work well. That 5.47 road-gear will get you about 60@4400, kindof a good place to be with a stock 318.
EDIT; I think someone once told me that not ALL A998s had the wide ratio gearset, so buyer beware. Mine does
Also, with a 2800TC, your 318 will haul the mail with this combo; and you can forget about up-camming it.