I don't think an engine swap that is done properly is going to hurt your resale value, if anything it might increase it since you would be going to a more desirable engine. What depreciates a car is a hack-job engine swap, things like wiring laying all over the place, a carburetor on a vehicle that was fuel injected, things like that.
As for the vortec heads, that refers to heads that came on vortec motors. The first thing you should know is that a chevy 4.3 is a chevy 5.7 with 2 cylinders lopped off. They have the exact same architecture, same bell housing pattern, same motor mount locations, same front accessory bracket locations, same water pump, same bore spacing, same bore and stroke, same intake and exhaust ports in the cylinder heads, etc. Vortec heads refers to the heads that were factory installed on the vortec motors, which were available in both V6 (4.3) and V8 (5.0 and 5.7). Installing vortec heads on a 4.3 will require installing the complete vortec intake setup, with sequential port injection, however since the V8 heads are plentiful and cheap and arguably the best factory head ever made for the small block chevy, numerous aftermarket intakes exist to allow you to run vortec heads on a chevy V8 with either a carburetor, or in your case, the TBI injection unit. So my suggestion would be buy a reman'd chevy 5.7 V8 with vortec heads, get the intake manifold to continue to use the TBI, wire the truck like a factory TBI V8 truck would have been (would be best to find a beat V8 donor truck for the wiring, ECM, and V8 TBI unit), and have a factory looking engine that makes double the horsepower, but will still idle and drive just as smooth, and last you another 30 years.
-91 Cougar LS, coming soon, complete overhaul
with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
-"Your buddy Mike is INSANE!" -ClintD's dad