Nitty-Gritty Swap Information:
Swapping an EA61 to an EA63:
Bolt-in, no mods necessary. Reuse your original intake, carb, and distributor. Some late EA63 used the wider bellhousing, so you might want to snag that and the starter too, just incase. Clutches and Pressure plates are identical. An evening job!
Swapping an EA61 to an EA71 or EA81:
You will need a complete EA71/81 to do the swap, as everything under the hood must be changed. This is a bolt-in swap. You will reuse the EA61 clutch disc, but the pressureplate, starter, flywheel, and bellhousing off of the EA71/81 must be used. A nice weekend project!
Swapping an EA63 to an EA71:
You will need a complete EA71 to do the swap, but this is a bolt-in job. If you get a motor with electronic ignition, you will have to reuse your old distributor or get a spark box. You must use an EA71 pressureplate, bellhousing, starter, and flywheel with your old EA63 clutch. An evening job!
Swapping an EA63 to an EA81:
You will need a complete EA81 to do the swap, and is identical to the EA71 swap with one exception: You must use a 2wd EA81 pressureplate, regardless if your car is 4wd or not. An evening job!
Swapping an EA71 to an EA81 or vice-versa:
100% Bolt-in. The only thing to watch out for is the distributors. You should always reuse the distributor that came with your original engine or you will have to rig up wiring and or make a Frankenstein distributor. I would do a full swap, but you can just swap the shortblock if you wish. Personally, I would use either block with the EA81 crank, rods, intake, 2bbl hitachi carb, and a 1983+ 2wd Nippondenso distributor, with an EA71 solid cam and lifters. (Best OEM combination you can put together).
Heads are up to you, if you have them shaved and ported/polished one is no better than the other. The EA81 head is 1″ longer but both will interchange with no problems. The only exception would be that in late 1983, EA81s started coming with hydraulic lifters instead of the solid one used for many years. You would have to have complete matching heads, cam, lifters, and pushrods to swap over to hydraulic but it is a bolt on feat, even for the EA71. 100% up to you, and really depends on how much you like that solid lifter sound and adjusting the valves, (not like its hard).
Honestly replacing an EA71 with an EA81 is silly, the last 1bbl EA71 and 2bbl EA81 that rolled off the assembly lines only have a 4HP and 11/lbft difference… throw the 2bbl on an 1981+ EA71 and grind the diffuser out of the exhaust port and they will be even… a thing to consider when doing a rebuild. An article on hot rodding the EA71/81 is soon to come!!