I'm going to be the black sheep here.
You have a possible 360 that's been rebuild with low miles so your not looking at a engine replacement in the near future.
If you want the put a 4 bbl intake and carb on, do it.
If you want a set of headers, get them.
Will it greatly improve the performance of your 360 or increase mpg, probably not, may see a small gain.
The plus side is should you decide to swap in a 390 or do a stroker build later, all of those things will bolt on the the new engine and that cost will have already have been absorbed.

As for the intakes, a Street Master needs a bit of port work to make it as good as some others, if you don't need the heat riser for cold weather I recommend the Performer RPM, if you need the heat riser pick up the Performer.
Some of the factory cast manifolds can be picked up cheaper, but anytime I can take 50 lbs off the engine I'll do it, that's also 50 lbs off the nose weight of the truck.

 

I know the OP said it's a rebuild with low miles but might want to run a leak down test or at least a compression test to gauge its health. That might help decide how deep to dig into it.

 

Just food for thought, if you do replace the intake, you have to take the heads off, and if you see no turkey pan (factory splash guard) laying in the block, I recommend getting one..... Means someone didnt put it back in on its last rebuild, keeps hot oil and heat off the underside of the intake and helps reduce fuel temps... wish i would have put one back in my truck, I didnt even know they existed until I seen a picture of a block being torn apart, but that was after I had mine back together , ive seen them before used on ebay and such... I say this because I fight with hot fuel on my truck and im sure the turkey pan would have helped... and im running the edelbrock performer 390.

 

Jakeharp, I think you meant to say take the valve covers off...heads can stay put if only swapping intakes.

If the OP uses the valley pan, just make sure it's not contacting the push rods. Few years ago I thought I had a bad lifter but turned out to be a push rod touching the valley pan and making it tap.

When swapping intakes, check the fit. Every so often they need to be machined a little to fit to prevent leaks.

 

Completely agree with this. I had a gutless 360, low compression, completely stock. I put headers on it, noticed a difference. I put a 4bbl carb on the stock 2bbl intake with an adapter, and STILL saw a difference. Was it enormous? No, but it was worth the effort. If I had an aluminum intake to put on it at the time, I would have certainly done it. I ran that thing for years, beat the crap out of it, and it never gave me any grief. And if I had known about the timing set difference, I would have done that as well.

If it's a keeper, and it's a low-miles rebuild, go for it

 

I stand corrected, its been over a year for me, you will however have to take the rocker assembly off in order to let the push-rods escape as you lift the intake off... Also another 'wish i had done that' recommendation, there is a video out there on a fella that really gives a great demonstration of how to massage the intake back onto the block seeing the bolts go in at an angle and can easily leak at the front and rear surfaces he demonstrates some easy torquing along with tapping from a mallet to pull that intake into position instead of just torquing and hoping for the best.... Also I recommend RTV for the front and rears rather than the cork seals.... only because i bought into it and have no leaks

Oh and dont get those rocker bolts mixed up.... theres a thinner bolt on each side for oil travel........ask how I know

 

I really appreciate all the input that has been given by everyone and now have to weigh the pros and cons. Thanks again for all the advice.