The 1450 rpm is on 50Hz power (1500 rpm synchronous speed for a 4-pole motor), so you can ignore that. Assuming it's a direct-drive blower, the operating speed will always be 1800 rpm for a 4-pole on 60Hz power (minus a few percent at full load). So will the blower speed, of course. So the only way to change air flow, and therefore power input to the blower wheel, is by choking off or opening up the air flow.
Power input to the blower wheel will decrease with air flow reduction, which is a property of centrifugal blowers and pumps*, so that's one way to get the current draw under control. And you absolutely do need to put a clamp-on ammeter on that blower and compare to the motor's nameplate FLA (full load amps) value.
Belt-drive blowers used to come with an adjustable motor sheave, where you could screw the outer half in or out to change the pitch diameter and thereby change the blower wheel speed. I'm not in HVAC, but I've always assumed that was to tune the motor to the load, since the duct work back pressure (and front pressure, from returns) is different for every installation, and an installer wouldn't want a new installation to overload the blower motor.
It sounds like your existing installation doesn't have enough static pressure to keep motor current under the rated value. That's not surprising if there's no ductwork. I salvaged the blower from my attic A/C system when it was replaced years ago, and when I ran it on the garage floor, it pulled something like twice the rated current. It ran in my attic through an extensive duct network for about 5 years that I lived there, and who knows how long before that. Obviously the flow resistance was adequate. Just an FYI.
Having said all that, I agree with Roly that as a general rule, the load is the load. Working a motor at a low output does put it in a lower efficiency than it would be at rated load, which is normally the highest efficiency or close to it by design, but for intermittent home shop use, it's not likely to make a noticeable difference. Large motors, like 500 hp and up, have extremely high efficiency (like 96% or more) and oddly enough, that doesn't change but a little across the output power range (1/4 to 5/4 output power).
Single-phase motors, however, are a funny animal. Low power motors have terrible efficiency, and the lower the rated power, the lower the efficiency and power factor (the percentage of current that drives the load and makes heat, but it's always less than 100%). So it's entirely possible, likely even, that a 3/4 hp motor operating at half power will have the same efficiency
or even higher as a 1/3 hp motor running at full power.
Just a caveat about the current draw, though - the 3/4 hp may draw more current even if it has higher efficiency, because power factor is likely to be lower when running under partial load. Your electric meter does not register this extra current (reactive power) and only registers the 'real' portion of the current**. So even though the current draw may be higher, your meter is not likely to spin any faster, and may even spin slower, than with the motor you're taking out.
So use the bigger motor and don't worry about it. And sorry for the diatribe, but I'm quarantined in my travel van in my driveway after coming back from another country on 'essential' business, and have absolutely nothing better to do. Though I
could do some work I suppose, but this is more fun, and it
is Labor Day weekend after all. I'm also re-watching the whole Breaking Bad series at the same time.
* Impulse fans and pumps, like a table fan or boat prop, work the opposite way; increase the static pressure, and power input increases.
** Watts (W) is 'real' power, and volt-amps (VA) is 'real' plus 'reactive' power, or apparent power. A simple ammeter reading multiplied by voltage is the apparent power. A Kill-A-Watt unit will display watts and volt-amps, along with power factor (VA x pf = W). VA is always equal (pure resistive load) or higher than (any reactive load, like motors, capacitors, transformers) W. The reactive component is out of phase with voltage, and is just sort of bounching back and forth between the load and the utility. Sometimes called 'magnetizing current' when talking about motors and transformers, it's a bit like pendulum where it's trading elevation and speed, back and forth, but not actually doing anything, even if it's a heavy pendulum that could take your head off if you got in the way (like a kid on a swing). Only commercial meters, and I guess so-called 'smart' meters, register VA, but only commercial users get charged for excessively low power factor. Not home owners. Unless that's changing these days with smart meters.