It has been my experience that if I can't find or don't want to pay a higher price for a 970 part --- I use the corresponding part available on the 990.
The 300CX loader is available for the 970-1070 tractor's. The 8A-8B BH's are very nearly identical --- after viewing Mr. Grizz's pics of his 8B on a 990 and comparing them to my 8A backhoe.
In my opinion, I would rather have a low-hour good condition JD 1070 before a new, JD 990. The snychro-tranny on the 1070 is a much better unit than the collar shift 990 transmission. In addition, the draft-sensing 3pt hitch on the 1070 is a better 3pt configuration, than the 3pt available on the 990.
If your 1070 is in good shape and it handles all the work you can throw at it --- hang on to it!
Sell or trade the model 80 FEL (they'll work on the older 50 series CUT's) and upgrade to the 300CX. (Another ~350 lbs lift and the quick-tach bucket.)
But, if your 1070 is gettin' a little long in the tooth and you can see some big dollars on the horizon to get her in tip top shape again --- move on up to the 4000-20 series. That would make more sense, IMO that shifting straight across to the 990.
Heck, as I write this, it occurs to me that you're keeping the 2 attachments that are most likely to need the most attention to repairs and maintainence --- the FEL and BH. Pins and collars get worn and sloppy. Cylinders start to weep and drip. Hydraulic hoses get weather checked and rusty.
The better approach might be to keep the 1070 and make a deal on a new 8B hoe and 300CX loader...
AKfish