The 301 is quite reliable when operated within its intended rpm range, and the engine seems to respond favorably to the same modifications hobbyists perform on traditional Pontiac V-8s. Its block and internal components were designed for output levels near the stock rating and redline of about 4,500 rpm, however. When pushed beyond those limits, catastrophic damage can result, and I think that's why the engine has such a bad reputation from performance enthusiasts.
When traditional Pontiac V-8 production ceased in 1978 (Pontiac 400 engines were stockpiled for the '79 model year), the 301 was the Division's lone V-8. Left without a performance mill for its '80 Trans Am, Pontiac engineers fitted the 301 with a turbocharger, which added about 60 hp but didn't increase its redline.
To withstand the increased output, the engine's critical stress areas were addressed. The crankshaft received deep-rolled fillets to increase rigidity and the block casting received extra material to strengthen it. Such blocks are identified by a 301T cast onto its side, and as far as I know, all 301 engines produced during the '80-'81 model year received this block casting, but the deep-rolled crankshaft was limited to Turbo engines.
If you plan on using a 301 as the foundation for a performance build, you'd be wise to further address the weak links. Since you have a 301T block available, I suggest replacing its original reciprocating assembly with suitable stock or aftermarket components. Since it contains a 3.00-inch main-journal-diameter, you may be able to use a stock-type crankshaft like your 400ci unit, but you'll need correct-length aftermarket connecting rods to accommodate the low-deck block, and most likely custom-made pistons. It can then be internally balanced and a normal harmonic balancer and flywheel used.
I believe the 301's potential was also limited by its lack of airflow. If you intend to extract a significant amount of power from it, I suggest incorporating the top-end components found on typical Pontiac V-8s. Traditional cylinder heads will bolt on, but because the coolant passages of the 301 are slightly different, some welding work is required. You might also be limited to small-valve castings like No. 46 to prevent clearance issues with the cylinder walls. None of this takes into account combustion chamber size and the resultant compression ratio.
In addition to cylinder head modifications, you'll also need to fashion an intake manifold to fit your application. Because the Pontiac V-8 is a 90-degree arrangement and the 301ci is a low-deck design, you'll be forced to shorten the runners of a traditional intake manifold and bring the flanges closer to the plenum. Such modifications can be performed to a stock manifold, but there may be better options from the aftermarket, depending upon your performance goals and intended usage.
The 301 utilizes a camshaft with the same dimensions as that of a typical Pontiac engine, so obtaining one with valve events that suit your engine's needs shouldn't be overly difficult. Since the camshaft is in the same relative position to the crankshaft, a normal timing chain can be used. Stock-replacement 301 pushrods or aftermarket custom-length units will be required.
A significant amount of modification may be required to adapt the T5 to the 301. In addition to that, you might also find the need to strengthen the T5 to withstand the added output. You might find yourself time and money ahead by opting for an aftermarket kit that includes a TKO-500. It's much stronger and may be included in a kit that allows for easy installation into your Firebird.