A couple of the better Pontiac guys are right here, no need to go further. Between Bill (WDCreech), Lynn (LATECh) and myself, we gotcha "covered".
400 HP from a pump-gas 400 Pontiac is no problem at all. It takes a little port work, good induction/exhaust and a strong valve train. The factory Q-Jet intake will make more power than Performer (this is not a Chevy, and doesn't "act" like one).
48s are not "Ram Air" heads. This is a common area of confusion. 48s are '69 "large valve, small chamber" heads. Good ones, no doubt, but only a relative "handful" ended up on Ram Air engines (366 HP). Most were on manual trans 350HP 400s. Some were on 350HO engines, as well. The term "Ram Air head" should be reserved for "round port" (exhaust) heads with small (72 CC) chambers. This is to avoid confusion.
The Tremac is a decent transmission, but the Pontiac makes a WHOLE LOT more low-end torque than the more modern engines. We've seen success and failure using them. The Richmond 5-speed is KNOWN to be able to take the power. The TK-900 is pretty good, too, having driven a '79 T/A with one of our 461s in it.
You don't need to "stroke" the engine to make power. A 400 Pontiac is no punk. In fact, that's the engine that made GTO's "reputation" (which is NOT getting beat). With a good set of connecting rods,the Pontiac is tough and powerful. However, stroking it makes for the performance of a 455 without the weakness of the 455 (block). 400 blocks are physically stronger than 455s simply because the "hole" (main tunnel) down the middle is smaller, leaving more material in the bulkheads where it's needed.
Butler Performance is a good shop. But Pontiac "stroker kits" are available form every parts warehouse in the country where Eagle products are sold. Nothing 'rare" about Pontiac performance parts, and for the most part, fall in between BBC and SBC for "price" (like the CID and power levels).
Have fun! Cool car, a GTO ragtop!