Long term I'd like to be rid of the 305 SBC in my '84 Trans Am for a 355 SBC, but still retain the accessories that came with my current engine (knock sensor, electric fan, ECM in general, etc.)

I know it wouldn't be a 'drop in and go' type of swap since I'm keeping the ECM, so what all would I need to do?

The only things that come to mind are tapping new holes for sensors if not so equipped already and getting a new prom (and even then I wouldn't know which one).

Is there any year or casting of 350 I should look for in particular?

 

1980-1985. And no "tapping of holes" is necessary...

 

There's not really any such thing. There are 350s that have had the first instance of bore maintenance (.030") done to them, which raises their CID to about 355. That doesn't really make it "a 355"; it's still REALLY a 350. Or, if you get a 350 that's been bored out by .060" which is about as far as they can reasonably be counted on to reliably go and which most of the ones floating around on the market have reached that point, your new 350 will have about 360 CID. Doesn't too much matter though, it's still "a 350" for all intents and purposes; the coupla extra CID resulting from the maintenance are irrelevant and immaterial. You don't really want "an engine" from that year range, since it would come with some sort of ULTIMATE CRAP heads; just, the block. Best heads to get would be either Vette L98 heads (aluminum, casting # 113) or late 60s / early 70s "double hump" castings such as 186, 492, or 041 since those have the accessory bolt holes you need. NOT 461 (double humps from 68 back) since those won't have the bolt holes for your accessories, and NOT 487 or 487X. Just yerbasic flat-top 350 with the 64cc DH heads with bolt holes. And of course, NOT 882, 993, 624, or other 70s smogger heads, and NOT 193 TBI heads from 87 up.

If you get a 350 in whatever state of bore maintenance, within the same year range (roughly 80 or so up to 85... the earlier blocks had the dipstick on the driver's side, butt they changed that over in the various different car & truck chassis over the course of several years starting in about 78) as your 84, you won't have to do a thing to anything. Look for block casting 207 or 209; there are others butt those are by far the most common. It will be identical in every way, externally, to your 84 305. All the same bolt holes etc. are in the same places. Your 305 oil pan, valve covers, etc. etc. etc. will all bolt right up. The knock sensor will go in the hole that those genius GM engineers installed in the SBC back in 1955; being clairvoyant, they knew that electronic controls were coming to engines soon enough, so to fool those morons at Frod and Xler as to their future-proofing, they cleverly disguised the hole for the KS as the pass side block drain hole. Similarly, they disguised the hole for the electric fan switch, as the same hole that would have the temp gauge sending unit in it, if it was installed on the driver's side of the engine; they just put a pipe plug in that hole in the pass side head. Clever, those GM engineers.

"New PROM", in 2025, is mostly a pipe dream. The chip that's in that ECM is a Motorola 6809 from the mid 70s, which is a one-time-programmable device. Not erasable and reprogrammable like even the mid 80s ones. The one way to get one would be, find somebody that bought the "kit" that GM offered over the counter to do the exact upgrade you're contemplating, including a ZZ3 and then later a ZZ4 engine; this kit came with the engine (duh), a suitably altered PROM, some STICKERZZZZZ!!!!, and a few other goodies. Last time that I knew it was offered would have been 96 or so. There were a few others on the market at one time, notably Hypertech, butt I'd recommend you steer clear of those.

That said, because of the extremely limited functionality of the computer controls in that system, you don't really need a PROM at all, as long as you keep the physical build of the motor within a certain reasonable range. That mostly means, a cam with no more than 218° or so of intake duration. As long as you don't get too greedy with that, there's enough mechanical flexibility in the carb to adapt to it. I even had the whole electronic setup on a 400 in my 83, with a Comp 282S (solid lifter) cam, and it worked mostly OK; never did really get it "right", butt I drove it for years like that. Used it as a work truck, put probably 80,000 miles on it that way. I'd recommend not bothering with an old-skool flat-tappet cam; my suggestion would be a roller-tappet Comp XR264HR with LS6 springs and the Comp "adapter" retainers (787).

Be aware also, your L69 has some of the best rods GM put in production cars, if not THE best, for some reason. They're known as "X" rods, no doubt because they have "X" cast into them. If you can do without it for awhile, use those in your hypothetical new 350, as the vast majority of 350s have inferior ones. Those rods plus a forged crank are a bulletproof combo as far as those parts can be made so.

 

That's literally everything I needed to know and more, thank you!