It all depends on how far you want to take your truck.
Typically, the 60e will handle about 450-475rwhp in our trucks.
With your setup, say 10psi, cam and small truck, I'd say your truck will be pushing no more than 450rwhp; on the edge of that 60e.
Take your truck to 5psi, live with 400rwhp, and I think the FLT 60e will be fine for a long time.
OR
Start planning for an 80e, install a shiftkit and other upgrades, buy a $900 converter and don't worry about it.

When my Avalanche trans went at 10psi (broke the shaft) i wasn't going to mess around with 60e's breaking, so I went with an FLT L5 80e and it hasn't had one issue at 8-10psi in a 5900lb truck. Now the rearend is a different story lol.
A stock 80e will hang with your setup for a long time, keep that in mind.

Give Snyders a call and see if he can find you an 80e, should be under $1K.
Figure about $1500 for misc parts, fabrication, etc. That's just about what you'll get for your used 60e, just a thought.

I vote 80e!