You will need an intake that uses the L30/31 Vortec bolt pattern of 4 per side bolts that from the outside of the intake face straight down. There is no cylinder head exhaust heat crossover as on pre 1996 engines, if you must have an EGR it is externally plumbed. There are that I’m a aware of no intake manifolds made for the Q-Jet that also bolt to these head’s. You either will need a square to wide bore adapter or convert to a square bore carb Holley or Edelbrock.
You well need a conventional distributor as the L30/31 distributor has no internal advance system it is wholly dependent on the ECM for the timing function.
The 96 engine most likely is not machined for a mechanical pump, you will have to install an electric pump if the machining is not complete.
Good news is the 96 engine uses a factory roller cam.
The coolant pump on the 96 and up L30/31 engine is a lot different from your 81 pump. It is reverse direction in turning its shaft for the serpentine drive and has considerably different bypass nipples for the EFI system on those engines. Your first decision will be whether or not to replace the 81’s vee-belts with a factory serpentine system. The 96 block may or may not include the bypass passage the 81 block and pump has, you need to check this out as it may require you perform minor surgery to block and/or pump depending on your decision of drive system.
The 96 crankshaft uses a one piece rear seal with a 3 inch bolt pattern for flywheel/flexplate, you will need to bring the 86 up fly or flex along. The clutch/ torque converter patten remains the same but there are two production patterns of these so you need to measure what the 81 you have is using. The 350 one crank uses an external bit of counterweight; for the 81 engine with two piece rear seal this is on the crank itself for the one piece seal engines this moves onto the flywheel or flexplate. This bit of weight may be part of the fly/ flex or not, when obtaining the appropriate fly/flex you need to be sure whether it is part of the unit or is a separate bolt on item.
Back on the front the 96 uses a plastic timing cover with a crank position sensor. Depending on whether the engine you’re using to swap has the needed timing marks on the cover and damper to set up a conventional distributor you may find it necessary to use the older all metal cover and a pre 96 damper.
So you need to make some decisions as to how you want to integrate the 96 engine into the 81 chassis, then get back with us for more in depth specifics that these swaps require. Nothing is really difficult but there are some details that need to be attended to it’s not a direct bolt for bolt swap.
Edelbrock Performer Vortec Intake Manifold for all Small-Block Chevrolet 262-400 c.i.d Engines with 1996+ L31 Vortec Cylinder Heads & all Modern Marine and Industrial Engines. Accepts Q-Jet or 4150-Style Square-Bore Carburetors. Non-EGR
And don't waste your money on the expensive aftermarket "carb conversion" Vortec intakes. Call up any boat repair/salvage yard and you can probably score one for $50.
The Marine world switched to Vortec heads but kept carbs for a long time, so they got licensing from GM to cast their own. It's the same intake used on Vortec crate engines for $400 but cast with a Penta, Mercruiser, or OMC emblem on the front. Very capable intake, very similar to performer. I used to have a stockpile of about 20 of them but they all got sold or used on my builds. I'm down to my last one and it's going on a 383 I'm building for a Baja Sunsport. The one that's on my Ebbtide even has the GM part number cast right into it beside a "Volvo Penta" casting.