Were there any changes made to the block of the 360 between the 1970s 360 blocks and a 1991 block? Will the heads and they'll train swap directly over onto a 91 block?

yes..the heads and everything will change over..the 91 block is still a LA block but has provisions for the Factory OEM hydraulic roller lifters and roller camshaft.

The flat tappet cam can still be used.

Cool. What do you mean by provisions? Are the lifter bores larger?

There is a provision for the lifter retainer bracket or whatever you call it . also pay attention to your oiling system up top with your head you have to use the right combination of pushrods and rocker arms when changing to a flat tappet as I recall.

Are any of the popular heads like the 452s the same height as the 1991 heads? So I could reuse the current valvetrain and just switch heads?

Why not keep the heads? Magnum heads outflow almost every LA head by a fair margain.
Also, if you plan to use headers with the "Magnum" series block, take note of the extra casting nubs on the sides of the block just below the decks. These interfere with the header pipes and don't allow the flanges to sit flush against the heads. Maybe mock it up out of the vehicle?

I think, but not sure, but isn't the cam to rocker shaft hole in block not drilled on the magnum engine? Magnums oil through the push rods and reg LA oil through the rocker shaft. Timing chain cover is different too, no place to bolt the fuel pump on and no cam eccentric. Can use an earlier cover, I did. An adapter is made for the FI cams to use a mech fuel pump.

Yes there was a brief year or two (iirc) transition period (or 2) on the blocks inbetween the std 70'80/90's LA blocks and the 93/94 mag blocks regarding the oil passages on either valley wall (maybe present but not drilled) the guys here know cuz I've read it prior. that is the main caveat, & if you do use the LA heads & there are no passages you can easily external oil them. EDIT With more thought, there might be something different in the roller/non roller cam situation. Hang loose, some one will help us (iirc it was Scratchnfotraction who is up on the ins and out of em.

I don't have a magnum engine, my 360 is one of the last years of the LA block and I don't know if it has the correct oiling setup 4 Magnum heads. Plus I'm not familiar with Magnum heads or pedestal rockers which I know wouldn't be too difficult to figure out. I guess I'm just more comfortable using head I'm more familiar with and that have more intake options. I'm not sure but I think I can pick up a pair of cheap 452 heads and get them cleaned up and some minor port work done cheaper then I can buy a set of Magnum heads. I know the Magnum heads have issues with cracking and I'm more confident that I can find a set of used older heads that don't have cracks. The only thing that I'm somewhat worried about is the exhaust ports in my current heads being in a different position then the older heads which might make finding headers that fit in my truck more difficult

3 bolts down the center of the lifter gallery that holds the spider in place that hold the dogbones in place that hold the hydraulic roller lifters in place.

A reminder, the 452 heads are BB heads

You're totally right I just realized that after my last post. What la heads are the best bang for the buck? Come to think of it, does anyone know the casting number of the heads that came on a 91 360? Are they better design than previous years? Maybe I can just get them worked on instead of getting different heads, if the intake bolts hole positions are the same too then I could use an eddy intake

I think the 308's are the best iron with (iirc) the 915's 2nd (& there is a BB 915 head). Both are LA. Mags are better but need different parts/some adaptations

Its a simple pre magnum roller block, use any of the 70-s gear including cam if you wish but much more bang for the buck going with the hyd roller cam.

lifter bores are slightly taller on a roller 360 block. three mounts in the lifter valley for the lifter hold down spider.

Other than that it's pretty much the same as any 74 and up 360 (main bearing changed in 74).

Best heads are the magnums, which oil thru the pushrods so the block oiling provisions are moot there. The heads your 91 came with are probably the best stock LA heads out there, but the magnums are enough better to warrant the change.

http://www.hotrod.com/articles/mopp-0308-adding-magnum-to-a-360-short-block/
http://www.hughesengines.com/TechArticles/11convertingmagnumstyleheadstoworkonthelablock.php

My thoughts to on the heads, aren't the 91 heads the so called swirl port head? One thing I like about the magnum head is no heat crossover. There are a few head places out there who sell them set up ready to go with either intake bolt pattern for $500 or less depending what there's done to them, junk yard heads complete go for $50-75 a head, cracking is a non issue IMO.

91 360 heads, swirl port and open chamber, meh.

Right now Jeg's is offering a good deal on new EQ Iron heads (Magnum Style) for $500.00 and change assembled each. Or if you want to stay with the LA style, there is a pair of freshly machined "915" SB heads on the Tacoma CL for $700.00.....your choice.