[unruhjonny]

You can get the interior restiched in the original vinyl.
Really, in my opinion that's the only way to go.
If you do decide to go to a custom interior, I'd guess that just about any kind of blue cloth interior would look cheezy very quickly - especially in a '70-73 TA. If you do go cloth, I'd really suggest converting the car to a black interior, because typically black looks the best over time, and takes away much less value than any other colour.
I actually have in my posession some correct blue vinyl for your car - I bought it from a local interior shop for cheap becasue it was leftover from a '70 Firebird they did.
Keep in mind, if you have an upholsterer redo your interior, the labor is exactly the same - it's only the cost of the material that changes - so in theory stiching it in the correct colour vinyl would cost the same as cloth.
You're in the same boat as a fair percentage of 70-73 Firebird owners - I too have to get my ('73) interior redone by a shop, because the fabric pattern & colour was not nearly popular enough for the aftermarket to mass produce.

You didn't mention if anything else on the engine (eg: intake/ heads/ carb/ distributor) was changed out at the time of the rebuild...
Presuming you reused all those components, having a '70 T/A with everything underhood except for headers and a replacement shortblock is not a huge detractor on value.
You could one up yourself by keeping an eye out for a date coded correct '70-400, and if you don't want to have a "wrong code" block in there, you could just machine the front of it (to remove the block code & engine number, as well as remove any corrosion that is typically observed around the water pump passages), that would put you dead even with a replacement block - just please don't pull a "corvette" and attempt to restamp your block - I would never advocate or suggest such a practice.

Replaceing '70 RAIII exhaust manifolds aren't nearly as expensive as a set of headers - so I don't know where you got that from (I had to replace my drivers side one, after repeated "repairs" put it to the point of being unfixable)...
I found one via the classifieds on classical pontiac. for a very good price (I actually bought a set, and sold the one I didn't need to another gentleman).
You can also use any 1968-1969 model year GTO D-port RA manifolds, I cannot recall the differnces in using the early manifolds in a '70 Firebird auto vs stick car... I did run a '68 model year GTO manifold in my Formula for a while (it was a loaner!) - it was definatly tighter than the OE part, but I got it in by removing the starter, and messing with it for a while to figure out how to angle it in just right. (I did not have to disconnect motor mounts and/or raise my engine to get the '68 model year part in place.)

'70 T/A's are definatly gaining in value - so doing the car right would be the only route I would suggest for you.