Something purpose built for use at RF is better in several ways,
but the irf510 works and it's cheap. ?I'm happy to defend it.
Especially for use on a $59 ssb transceiver.

Many early attempts at using the switching fets fell short above 7mhz,
but the wa2eby amp gives a recipe for something that works to 30mhz.
Allison's 4x4 push pull using 8 irf510's for 225 watts on 50 mhz shows that
if you know what you are doing you can push it even further.

Yes, you need to account for the irf510's lousy heat transfer characteristics.?
The bond wires to the die add inductance that limit the max usable frequency.
The relatively high input capacitance (for an RF FET) means the driver stage
has to be very low impedance and give a fair bit of power as he frequency goes up.
The Vgs vs Id curves don't look terribly linear compared to a good RF FET.
RF amplifiers can be puzzling beasts even with the best of devices.

But there's many examples of good irf510 amps out there to copy from.
Be sure to look hard at the physical design too (short leads, ground planes, heat sinks,
input separated from output) if you expect similar results. ?
The pcb layout of the bitx40v3 probably won't work well?on the higher bands. ?
A push-pull amp has cleaner output and lower bias currents than the ?single irf510
of the bitx40, but a well adjusted bitx40 works surprisingly well.

The irf610 is very interesting, spec shows about the same gate charge as the irf510.
Curious that few if any amateur linears are are using it.

Here's a few links that might be of interest:

Jerry, KE7ER

Jerry:
I wouldn't be so quick to defend the IRFxxx series.
Remember, these were designed to be switching
transistors working at about 500 KHz.

Yes, they can be adapted to RF with great success, but,
as Allison, Motorola, and WA2EBY discovered, layout is
critical as well as heat dispersion.

Most of the higher output amps involve more than one
device with large compensation for the input capacitance,
and a heavy heat sink.

Incidentally, the IRF610 gives much the same performance
as the IRF510, but it is rated to 100v instead of 70v. It is
an easy fix for more power.

john
AD5YE